Compare EPUB, MOBI, PDF, AZW3, and other ebook formats. Covers reflowable vs fixed layout, DRM support, reader compatibility, multimedia features, and self-publishing requirements.
The ebook market has grown into a multi-billion dollar industry, and understanding ebook formats is essential whether you are a reader choosing between devices, an author self-publishing your first novel, or a publisher distributing content across platforms. Each format was designed with different priorities, and choosing the wrong one can result in broken layouts, limited distribution, or frustrated readers.
This guide covers every major ebook format in depth, comparing their features, compatibility, and ideal use cases so you can make an informed decision.
EPUB is the open standard maintained by the W3C (previously the IDPF). It is the most widely supported ebook format, accepted by virtually every ebook store and reading application except Amazon Kindle.
How it works: An EPUB file is essentially a ZIP archive containing XHTML content files, CSS stylesheets, images, fonts, and metadata. The .epub extension is just a renamed .zip file that follows a specific internal structure.
EPUB 2 vs EPUB 3:
| Feature | EPUB 2 | EPUB 3 |
|---|---|---|
| Content format | XHTML 1.1 | HTML5 |
| Styling | CSS 2.1 (subset) | CSS3 |
| Audio/Video | Not supported | HTML5 media elements |
| JavaScript | Not supported | Limited support |
| Fixed layout | Not supported | Supported |
| Accessibility | Basic | WCAG/ARIA integration |
| MathML | Not supported | Supported |
| Vertical text | Not supported | Supported (CJK) |
| Specification year | 2007 | 2011 (updated 2023) |
EPUB 3 is now the standard for new publications, though EPUB 2 files still work on most readers. If you are creating ebooks, always target EPUB 3 unless you have a specific reason to use the older version.
MOBI was the original Kindle format, inherited when Amazon acquired Mobipocket in 2005. It has been officially deprecated by Amazon, which now uses the KF8/AZW3 format. However, MOBI files are still widely shared and remain compatible with older Kindle devices.
Limitations: No support for HTML5, limited CSS support, no embedded fonts, no audio or video, and a maximum file size of 64 KB per record (though records chain together). MOBI files tend to be larger than equivalent EPUB files due to less efficient compression.
AZW3 is Amazon's current ebook format, based on HTML5 and CSS3. It offers significantly better formatting than MOBI while remaining exclusive to the Kindle ecosystem. Amazon typically wraps AZW3 content inside a MOBI container for backward compatibility.
Key features: Embedded fonts, CSS3 support (including Flexbox), fixed-layout support, SVG graphics, and better typographic control than MOBI. However, AZW3 files can only be read on Kindle devices and apps.
PDF is a fixed-layout format, meaning every page has an exact, predetermined appearance. This makes PDF excellent for documents where layout precision matters (textbooks, graphic novels, technical manuals) but problematic for typical reading on small screens.
Ebook limitations: PDF text does not reflow to fit different screen sizes. Reading a PDF formatted for A4 paper on a 6-inch Kindle screen requires constant zooming and panning. PDFs also lack ebook-specific features like adjustable font size, dictionary lookup, and reading progress synchronization.
That said, PDF remains the preferred format for certain types of content. Our in-depth EPUB vs PDF comparison covers when each format is the better choice.
KFX — Amazon's latest Kindle format, succeeding AZW3. Features enhanced typesetting (better hyphenation, kerning, drop caps) and page-flip preview. Only available through Amazon's publishing pipeline.
iBooks (IBA) — Apple's proprietary format created by iBooks Author. Supports rich media (video, 3D objects, interactive widgets) but is exclusive to Apple devices. Apple discontinued iBooks Author in 2020 in favor of Pages for EPUB export.
FB2 (FictionBook) — An XML-based format popular in Russia and Eastern Europe. Supports structured metadata and semantic markup but has limited adoption outside its core regions.
DJVU — Optimized for scanned documents with high-resolution images. Not a true ebook format, but commonly used for digitized books and academic papers.
This comprehensive comparison covers the features that matter most when choosing an ebook format:
| Feature | EPUB 3 | MOBI | AZW3/KF8 | KFX | |
|---|---|---|---|---|---|
| Reflowable text | Yes | Yes | Yes | No | Yes |
| Fixed layout | Yes | No | Yes | Yes (default) | Yes |
| Custom fonts | Yes | No | Yes | Yes (embedded) | Yes |
| Audio embedding | Yes | No | No | No | No |
| Video embedding | Yes | No | No | No | No |
| JavaScript | Limited | No | No | Limited | No |
| MathML | Yes | No | Partial | Via LaTeX | No |
| SVG graphics | Yes | No | Yes | No (rasterized) | Yes |
| DRM support | Adobe, LCP | Amazon | Amazon | Adobe | Amazon |
| Max file size | No limit | ~650 MB | ~650 MB | No limit | ~650 MB |
| Accessibility | Excellent | Poor | Moderate | Variable | Moderate |
| Open standard | Yes (W3C) | No | No | Yes (ISO) | No |
Pro Tip: If you need to distribute the same content across multiple platforms, author in EPUB 3 and convert to other formats as needed. EPUB's open standard and rich feature set make it the best source format for multi-platform publishing. Use our document converter to handle format transformations.
Understanding which devices and apps support which formats is critical for reaching your audience.
| Reader/App | EPUB | MOBI | AZW3 | KFX | |
|---|---|---|---|---|---|
| Amazon Kindle (e-ink) | No | Yes | Yes | Yes | Yes |
| Kindle App (iOS/Android) | No | Yes | Yes | Yes | Yes |
| Apple Books | Yes | No | No | Yes | No |
| Google Play Books | Yes | No | No | Yes | No |
| Kobo | Yes | Yes | No | Yes | No |
| Nook | Yes | No | No | Yes | No |
| Calibre | Yes | Yes | Yes | Yes | No |
| Readium | Yes | No | No | No | No |
| Adobe Digital Editions | Yes | No | No | Yes | No |
| Moon+ Reader (Android) | Yes | Yes | No | Yes | No |
| Libby/OverDrive | Yes | No | No | No | No |
The key takeaway: EPUB works everywhere except Kindle, and AZW3/KFX work only on Kindle. This fundamental split drives most format decisions in publishing.
Calibre is the gold standard for ebook format conversion. It handles EPUB to MOBI, EPUB to AZW3, PDF to EPUB, and virtually every other combination. The conversion quality varies by format pair:
For quick conversions without installing software, our EPUB converter handles the most common format transformations. If you need to convert between EPUB and MOBI specifically, our guide on converting EPUB to MOBI covers the process and pitfalls in detail.
This is the most important architectural distinction in ebook formats, and it determines how your content adapts to different screens and reading preferences.
Reflowable ebooks adapt their layout to the reader's screen size, font size preference, and reading orientation. The text wraps, images resize, and the pagination adjusts automatically.
Best for:
Limitations:
Fixed-layout ebooks present each page as a designed canvas with exact positioning of text, images, and graphics. The reader can zoom but cannot change the layout.
Best for:
Limitations:
Pro Tip: If your book has mostly text with occasional images, use reflowable layout. Only choose fixed layout when the visual design of every page is essential to the reading experience, like in a graphic novel or photography book.
Each major ebook platform has specific format requirements for authors uploading their books.
Accepted formats: EPUB (preferred), DOCX, KPF (Kindle Package Format), PDF (discouraged)
Amazon converts your uploaded file to its proprietary formats internally. Starting in 2023, Amazon recommends uploading EPUB files for the best conversion quality. The older workflow of uploading MOBI is now deprecated.
Key requirements:
Accepted formats: EPUB 3 (preferred), EPUB 2
Apple has strict EPUB validation requirements. Files must pass epubcheck with zero errors. Apple Books supports the widest range of EPUB 3 features, including embedded audio, video, and JavaScript widgets.
Accepted formats: EPUB, PDF
Google accepts both EPUB and PDF but recommends EPUB for reflowable content. PDF books are listed as "scanned pages" and receive less favorable placement in search results.
Accepted formats: EPUB 3, EPUB 2
Kobo is fully EPUB-centric and has excellent support for EPUB 3 features, including embedded fonts, advanced CSS, and fixed layout.
| Platform | Preferred Format | Cover Min Size | Max File | DRM |
|---|---|---|---|---|
| Amazon KDP | EPUB | 2560 x 1600 px | 650 MB | Amazon (automatic) |
| Apple Books | EPUB 3 | 1400 x 1873 px | 2 GB | FairPlay (optional) |
| Google Play | EPUB | 1600 x 2400 px | 2 GB | Adobe (optional) |
| Kobo | EPUB | 1400 x 1873 px | 700 MB | Kobo (optional) |
| Smashwords | EPUB | 1600 x 2400 px | 5 MB | None |
| Draft2Digital | EPUB | 1600 x 2400 px | 200 MB | Retailer-specific |
The simplest workflow for a novel or short story collection:
Our tutorial on creating EPUBs from Word documents walks through this process step by step.
Non-fiction books with images, tables, and complex formatting need more attention:
Technical books with code listings, math equations, and diagrams:
DRM protection is one of the most debated topics in ebook publishing. Here is what you need to know:
Most self-publishing experts recommend against DRM for independent authors. DRM does not prevent piracy (DRM removal tools are widely available) but does frustrate legitimate buyers who want to read on multiple devices. Watermarking (social DRM) provides accountability without compatibility friction.
Major publishers continue to use DRM largely due to contractual obligations with authors and agents, not because it effectively prevents unauthorized copying.
Large ebook files download slowly, consume device storage, and may exceed platform upload limits. Here are strategies for keeping file sizes manageable:
Images are almost always the largest component of an ebook. Optimize them before embedding:
Use our image compressor to reduce image file sizes before embedding them in your ebook. For a deeper dive, our guide on lossless vs lossy compression explains the tradeoffs.
Embedded fonts add 50-500 KB each to your ebook. Subset fonts to include only the characters actually used in your text. Tools like pyftsubset (part of fonttools) can reduce a 200 KB font file to under 30 KB.
Ebook accessibility is not just a legal requirement (it is increasingly mandated by law in the EU and other regions) — it also reaches a wider audience and improves the reading experience for everyone.
alt attributes to all imageslang attributes for multilingual contentFor most authors and publishers in 2026, the decision comes down to this:
If you need to convert between ebook formats, our document converter and EPUB converter handle the most common transformations. And for a focused comparison of the two most popular formats, our EPUB vs PDF guide provides detailed analysis of when each format excels.
Emma Wilson
Digital media specialist with expertise in audio engineering, podcast production, and ebook publishing workflows.