How to Annotate PDF Online: Highlight, Comment & Mark Up

Why Annotate a PDF?

Reading a document passively is one thing. Engaging with it -- highlighting key passages, scribbling notes in the margins, circling important figures, flagging sections for follow-up -- is how real work gets done. Annotations transform a static PDF from something you read into something you actively think with.

The challenge is that PDFs were not designed for editing. They were designed to look identical everywhere, which means they resist modification by nature. But annotation is different from editing. You are not changing the original content -- you are adding a layer of commentary on top of it. This distinction is important because it means PDF annotations are technically separate objects that sit above the page content, which is why they can be added, removed, or modified without altering the underlying document.

In practice, PDF annotation covers a wide range of activities: highlighting text for emphasis, adding sticky-note comments with questions or feedback, drawing shapes to circle or underline important areas, inserting text boxes with corrections or clarifications, and stamping documents with approval or review status. Whether you are a student marking up a research paper, a lawyer reviewing a contract, a designer giving feedback on a layout, or a team reviewing a proposal together, PDF annotation is the standard workflow.

This guide covers every annotation type, the tools available (free and paid), collaborative annotation workflows, and best practices for keeping your markups organized and useful.

Annotation Types Explained

Before diving into specific tools, it helps to understand what kinds of annotations are available. Not every PDF tool supports every type, and knowing the terminology helps you choose the right tool for your needs.

Highlight, Underline, and Strikethrough

These are the most common text markup annotations. They work by selecting text on the page and applying a visual effect -- a colored background (highlight), an underline, or a strikethrough line. Most tools let you choose the color, which is useful for creating a color-coded system: yellow for important, green for agreed, red for needs attention.

The key limitation is that these annotations only work on PDFs with real, selectable text. Scanned PDFs (which are just images of pages) do not have selectable text, so text markup tools will not work on them unless the document has been processed with OCR first.

Sticky Notes and Comments

Sticky notes are the digital equivalent of Post-it notes attached to a document. They appear as small icons on the page (usually a speech bubble or note icon) and expand when clicked to reveal a text comment. This makes them ideal for adding feedback without cluttering the page visually.

In collaborative workflows, sticky notes typically show the author's name and timestamp, creating a conversation thread. Multiple reviewers can add their own notes, and the document owner can reply to each one. This makes sticky notes the backbone of PDF-based document review.

Shapes and Freehand Drawing

When you need to point to something that is not text -- an image, a chart area, a layout element, or a specific spot on a diagram -- shapes and freehand drawing let you mark up visually. Rectangles can frame areas of interest, circles can highlight specific data points, lines and arrows can connect related elements or point to problems.

Freehand drawing is particularly useful on tablets with stylus support. Architects, engineers, and designers frequently annotate PDF drawings and plans with freehand markups, using the stylus to draw directly on the document just as they would with a pen on paper.

Method 1: Annotate PDF Online with ConvertIntoMP4

The fastest way to annotate a PDF without installing any software is through a browser-based tool. The PDF editor on ConvertIntoMP4 supports the most common annotation operations directly in your browser.

Step-by-Step Walkthrough

Step 1: Open the editor. Navigate to the PDF editor in any modern browser. No account is required to get started.

Step 2: Upload your PDF. Click the upload area or drag your PDF file directly onto the page. The document loads in the browser with a toolbar of annotation tools.

Step 3: Choose your annotation tool. The toolbar provides options for text, rectangles, circles, highlights, and freehand drawing. Select the tool you need.

Step 4: Add your annotations. Click or drag on the document to place annotations. For text annotations, type your comment. For shapes, click and drag to set the size and position. Adjust color and size settings as needed.

Step 5: Save and download. When your annotations are complete, click the save or download button. The annotated PDF downloads to your device with all markups embedded in the file.

The PDF editor is ideal for quick annotation tasks -- marking up a contract, adding notes to a report, or commenting on a design proof. For more extensive editing like adding or removing content from the PDF itself, see our guide on how to edit a PDF online.

Method 2: Using Adobe Acrobat Reader (Free)

Adobe Acrobat Reader -- the free version, not Acrobat Pro -- includes a surprisingly capable set of annotation tools. Since most computers already have it installed, this is often the most accessible option for desktop annotation.

Available Annotation Tools in the Free Reader

• Highlight, underline, strikethrough for text markup
• Sticky notes for comments
• Text boxes for visible text additions
• Drawing tools (pencil, shapes, lines, arrows)
• Stamp tool with predefined stamps (Approved, Draft, Confidential, etc.)

How to Annotate in Acrobat Reader

1. Open your PDF in Adobe Acrobat Reader
2. Click the Comment tool in the right-side panel (or go to View > Tools > Comment > Open)
3. The annotation toolbar appears at the top of the document
4. Select your annotation tool and click on the document to place it
5. For text markup, select the text first, then right-click and choose your markup type
6. Save the file with File > Save (annotations are saved directly into the PDF)

Limitations of the Free Reader

The free Reader does not support editing existing text, adding or removing pages, redacting content, or creating forms. It also lacks the advanced comment management features in Acrobat Pro, such as comment filtering by reviewer and batch comment operations. For these advanced needs, you would need either Acrobat Pro or a tool like our PDF editor.

Method 3: Using Preview on macOS

Mac users have a solid PDF annotation tool built directly into the operating system. Preview, the default PDF and image viewer on macOS, includes a markup toolbar with annotation features that cover most common needs.

How to Annotate in Preview

1. Open your PDF in Preview (right-click the file, select Open With > Preview)
2. Click the Markup toolbar button (pen-tip icon) or press Shift+Command+A
3. Use the toolbar to access:
   • Text selection with highlight, underline, and strikethrough
   • Shapes (rectangles, circles, lines, arrows, speech bubbles)
   • Text boxes for adding visible text
   • Freehand drawing (sketch tool)
   • Signatures (if you need to sign while annotating)
   • Color and border adjustments for all markup elements
4. Save with Command+S to embed annotations in the PDF

Preview is fast, free, and handles most annotation scenarios. The main limitation is that it does not support sticky-note-style comments with collapsible text -- your text annotations are always visible on the page. For documents where you need hidden comments that expand on click, use Acrobat Reader or an online tool.

Method 4: Using Browser Extensions and Web Apps

Several browser-based annotation tools provide full-featured markup without installing desktop software.

Hypothes.is (Collaborative Web Annotation)

Hypothes.is is a free, open-source tool designed for collaborative annotation. It works as a browser extension and supports annotating any web page or PDF viewed in the browser. Annotations are stored online and can be shared with groups, making it popular in academic and research contexts.

Kami (Education-Focused)

Kami is a browser-based PDF and document annotation tool widely used in education. It integrates with Google Classroom and provides real-time collaborative annotation. Students and teachers can annotate the same document simultaneously, with each person's markups appearing in a different color.

Xodo (Cross-Platform)

Xodo works across web, desktop, and mobile platforms and offers a comprehensive set of annotation tools including text markup, shapes, stamps, freehand drawing, and sticky notes. The free tier covers basic annotation, while the paid tier adds features like document comparison and batch processing.

Collaborative Annotation Workflows

Annotating a document alone is useful. Annotating with a team is transformative. Collaborative annotation turns a static document into a conversation, where multiple reviewers can contribute feedback, respond to each other's comments, and track the status of every annotation.

Setting Up a Review Workflow

The most effective collaborative annotation workflows follow a clear structure:

1. Distribute the PDF to all reviewers, either by sharing a link (for cloud-based tools) or by sending the file
2. Assign reviewer roles so each person knows what to look for -- content accuracy, legal compliance, design quality, typos and grammar
3. Use consistent annotation conventions (color coding, stamp usage, comment tagging)
4. Set a review deadline and compile annotations when all reviewers have finished
5. Resolve annotations by accepting or rejecting each comment and making the corresponding changes

Managing Multiple Reviewers

When multiple people annotate the same PDF, keeping track of who said what requires organization.

Annotation Best Practices

Be Specific and Actionable

Vague annotations like "Fix this" or "Not sure about this section" waste the reader's time. Instead, write annotations that clearly state the issue and suggest a solution: "This paragraph contradicts the data in Table 3 on page 12 -- verify the 2025 figures" or "Consider moving this section before the methodology section for better flow."

Use the Right Annotation Type

Do not use a text box when a sticky note would be better. Text boxes are visible on the page and clutter the layout; sticky notes collapse to a small icon and keep the page clean. Use text boxes for corrections that should be visible (like adding a missing label to a chart) and sticky notes for comments and feedback.

Create a Color-Coding System

If you annotate documents regularly, establish a personal (or team-wide) color system:

• Yellow -- General notes, points of interest
• Green -- Approved, confirmed, or verified
• Red -- Needs revision, error, or disagreement
• Blue -- Questions for the author
• Orange -- Suggestions (optional changes)

Layer Your Annotations

Start with a first pass where you read the entire document and add high-level annotations. Then do a second pass focused on detail -- grammar, accuracy, formatting. This prevents you from getting bogged down in typos before you have assessed whether an entire section needs to be rewritten.

Annotations and Flattening

One important consideration with PDF annotations is that they exist as a separate layer on top of the page content. This means they can be removed, modified, or hidden by anyone who opens the file in an annotation-capable viewer. If you need your annotations to become permanent and uneditable -- for example, when sharing a reviewed document externally -- you need to flatten the PDF.

Flattening merges all annotations into the page content, making them permanent. After flattening, highlights, comments, stamps, and drawings become part of the static page image and cannot be selected, edited, or deleted. For a detailed guide on this process, see our article on how to flatten a PDF.

You can flatten a PDF using the PDF editor on ConvertIntoMP4, Adobe Acrobat Pro, or various other tools. The key decision is whether to flatten all annotations or selectively flatten only certain types -- some tools allow you to keep form fields interactive while flattening all other annotations, for example.

Annotating Scanned PDFs

Scanned PDFs -- documents created by scanning paper through a photocopier or scanner -- present a unique challenge for annotation. Because these PDFs are essentially images of pages rather than actual text, text markup tools (highlight, underline, strikethrough) do not work. The annotator cannot select text because there is no text to select.

There are two approaches:

Annotate visually. Use shapes, freehand drawing, and text boxes to mark up the scanned pages. You can draw a rectangle around a paragraph, add a text box with your comment, or use the pencil tool to circle important areas. This works but is less precise than text-based annotation.

Run OCR first. Process the scanned PDF through an OCR tool to convert the page images into real, selectable text. Once the PDF has an OCR text layer, all annotation tools work normally -- you can highlight specific words, add sticky notes anchored to specific text passages, and use strikethrough to suggest deletions. See our complete guide on how to OCR scanned documents for step-by-step instructions.

Annotations on Mobile

Annotating PDFs on phones and tablets is increasingly common, especially for people who review documents while commuting, traveling, or working away from their desk.

iOS (iPhone and iPad)

The Files app on iOS opens PDFs with a built-in markup toolbar. Tap the pencil icon to access drawing tools, text boxes, shapes, and signatures. The Apple Pencil on iPad makes freehand annotation particularly natural. For more advanced annotation, apps like PDF Expert and GoodNotes provide comprehensive annotation features with cloud sync.

Android

Android's default PDF viewer varies by manufacturer and is often limited. Google Drive can open and display PDFs with basic annotation support. For serious annotation on Android, use a dedicated app like Xodo, Adobe Acrobat Reader, or Foxit PDF Editor, which provide full annotation toolsets comparable to desktop software.

Online tools like the PDF editor also work in mobile browsers, giving you a platform-independent option that does not require installing an app.

Common Annotation Scenarios

Academic Paper Review

Peer review of academic papers relies heavily on PDF annotation. Reviewers highlight passages they find unclear, add sticky notes with substantive feedback, use strikethrough to suggest text deletion, and add text boxes with proposed rewording. For heavy revision, it may be more efficient to convert the PDF to Word, make changes with track changes, and then convert back to PDF.

Legal Document Review

Legal professionals annotate contracts, briefs, and case documents with extreme precision. Every annotation needs to reference specific clauses and include clear, unambiguous language. Signing a PDF is often the final step after the annotation and review cycle is complete.

Design and Layout Feedback

When reviewing PDF proofs of brochures, posters, or publications, annotations focus on visual elements: color accuracy, image placement, text alignment, and margin consistency. Freehand drawing and shape tools are more useful here than text markup, since the feedback is about visual layout rather than written content. Adding a watermark with "DRAFT" or "PROOF" status helps prevent annotated proofs from being mistaken for final versions.

Exporting and Sharing Annotations

Exporting a Summary

Most professional PDF tools can export an annotation summary -- a separate document listing every annotation with its page number, type, author, date, and text content. This is invaluable for tracking review progress and ensuring every comment has been addressed.

In Adobe Acrobat, go to Comment > Create Comment Summary to generate a printable summary. In other tools, look for an "Export Annotations" or "Comment Summary" option.

Sharing Annotated PDFs

When sharing an annotated PDF, consider your audience:

• Internal team review: Share the PDF with live annotations so reviewers can respond to comments and add their own feedback
• Client or external sharing: Flatten the annotations first so they cannot be modified, or export a comment summary as a separate document
• Archival: Flatten all annotations to create a permanent record of the review, then store alongside the clean final version

For documents that need to be shared as images rather than PDFs, you can convert the annotated PDF to JPG or PNG to capture the annotations as part of the page image.

Frequently Asked Questions

Can I annotate a PDF without Adobe Acrobat?

Yes. Multiple free options exist: the ConvertIntoMP4 PDF editor works in any browser, macOS Preview handles basic annotations, and free apps like Xodo and Foxit Reader provide comprehensive annotation tools. You do not need Acrobat or any paid software for standard annotation tasks.

Are PDF annotations permanent?

By default, no. Annotations are stored as a separate layer and can be edited or deleted by anyone who opens the file in an annotation-capable tool. To make annotations permanent, you need to flatten the PDF, which merges annotations into the page content. See our guide on how to flatten a PDF for details.

Can multiple people annotate the same PDF?

Yes, using either sequential or simultaneous workflows. Cloud-based tools like Kami and Hypothes.is allow real-time collaborative annotation. Alternatively, multiple people can annotate separate copies, and the annotations can be merged afterward. See the "Collaborative Annotation Workflows" section above for details on each approach.

How do I remove annotations from a PDF?

In most PDF tools, you can select an annotation and press Delete to remove it. To remove all annotations at once, use Adobe Acrobat's Comment > Delete All Comments option, or use a tool that supports batch annotation removal. Note that this only works if the annotations have not been flattened -- once flattened, they are permanently part of the page.

Can I annotate a scanned PDF?

You can use visual annotation tools (shapes, text boxes, freehand drawing) on any PDF, including scanned ones. However, text-based annotations (highlight, underline, strikethrough) require selectable text. For scanned PDFs, run OCR first to create a text layer, then annotate normally.

Wrapping Up

PDF annotation is a core skill for anyone who works with documents professionally. The tools are mature, widely available, and mostly free. Whether you use a browser-based tool like the PDF editor, the free Adobe Acrobat Reader, macOS Preview, or a mobile app, the key to effective annotation is not the tool itself but the workflow around it: be specific in your comments, use consistent conventions, and choose the right annotation type for each situation.

For quick markup tasks, an online tool gets the job done in minutes with no installation required. For regular, heavy annotation work, invest time in learning one tool well and establishing a color-coding and commenting system that keeps your feedback organized and actionable. Your future self -- and your colleagues -- will thank you for the clarity.