eBook Formats Explained: EPUB vs MOBI vs PDF
A comprehensive guide to EPUB, MOBI, AZW, and PDF eBook formats. Compare features, device compatibility, DRM, file sizes, and learn which format is best for your reading device in 2026.
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Choosing the right eBook format can make or break your reading experience. Whether you're a Kindle loyalist, an Apple Books fan, or someone who reads across many devices, the format you choose determines text clarity, device compatibility, portability, and whether you can even open the file at all.
This guide breaks down the three most common eBook formats — EPUB, MOBI/AZW, and PDF — covering their technical underpinnings, strengths, weaknesses, DRM implications, and the best use case for each. We'll also cover newer formats like KFX and FB2 for completeness.
What Is an eBook Format?
An eBook format is a file standard that defines how digital text, images, metadata, and styling are stored and rendered on reading devices and apps. Different formats have different capabilities: some reflow text to fit any screen, others preserve a fixed page layout, and some are locked to specific ecosystems through Digital Rights Management (DRM).
The three dominant formats today are EPUB (the open standard), MOBI/AZW (Amazon's proprietary formats), and PDF (Adobe's fixed-layout document format).
EPUB: The Open Standard
EPUB (Electronic Publication) is maintained by the W3C and is the most widely adopted eBook format outside of Amazon's ecosystem. It is based on web standards — HTML, CSS, and XML — which makes it flexible and future-proof.
EPUB Versions
- EPUB 2 — the legacy version, supports basic HTML and CSS. Still widely used.
- EPUB 3 — the current standard, supports HTML5, CSS3, JavaScript, embedded audio/video, MathML, and accessibility features like ARIA landmarks and media overlays.
Device and App Compatibility
EPUB is natively supported by Apple Books, Google Play Books, Kobo, Barnes & Noble Nook, Calibre, and virtually every non-Amazon eReader app. Amazon Kindle does not natively support EPUB — you need to convert EPUB to MOBI or use Kindle's "Send to Kindle" feature which converts to KFX internally.
Key Strengths
- Reflowable text — content adapts to any screen size, from a phone to a 10-inch tablet
- Open standard — not controlled by any single company, widely documented
- Rich media — EPUB 3 supports audio, video, interactive elements, and JavaScript
- Accessibility — built-in support for screen readers, text-to-speech, and alternative text
- Small file size — text-heavy EPUBs are typically 1-5 MB
Limitations
- Complex layouts (tables, multi-column text) may render inconsistently across readers
- No native Kindle support — requires conversion
- DRM varies by vendor (Adobe DRM, Apple FairPlay, etc.)
MOBI and AZW: Amazon's Kindle Formats
MOBI (Mobipocket) was the original format for Kindle devices. Amazon acquired Mobipocket in 2005 and built Kindle around it. AZW is Amazon's DRM-protected extension of MOBI, and AZW3 (also called KF8) added support for HTML5 and CSS3 — essentially Amazon's answer to EPUB 3.
The Evolution
- MOBI — original format, basic HTML/CSS, limited formatting
- AZW — MOBI with Amazon DRM
- AZW3 / KF8 — significant upgrade with HTML5/CSS3 support, embedded fonts
- KFX — Amazon's latest format, supports enhanced typesetting, Bookerly font, page flip, X-Ray, Word Wise
Device Compatibility
MOBI and AZW work on all Kindle devices and the Kindle app on iOS, Android, Mac, and PC. They do not work on Apple Books, Kobo, or other non-Amazon platforms.
If you have MOBI files and want to read them elsewhere, you can convert MOBI to EPUB or convert MOBI to PDF.
Key Strengths
- Native Kindle integration with all features (highlights, X-Ray, Word Wise)
- DRM support protects purchased content
- Good text reflowing on Kindle devices
- Whispersync support for syncing reading position across devices
Limitations
- Proprietary to Amazon — useless outside the Kindle ecosystem
- MOBI is being deprecated in favor of KFX; Amazon stopped accepting MOBI uploads in 2023
- Limited formatting compared to EPUB 3
- Cannot be read by most third-party eReader apps
PDF: The Fixed-Layout Universal Format
PDF (Portable Document Format) was created by Adobe in 1993 and is now an ISO standard. Unlike EPUB and MOBI, PDF maintains an exact, pixel-perfect page layout regardless of the device or screen size.
How PDFs Differ from eBooks
PDFs are technically documents, not eBooks. They don't reflow text, don't adapt to screen sizes, and don't support eReader features like bookmarks syncing or Word Wise. However, they excel when layout matters — think textbooks with diagrams, sheet music, manga, or academic papers.
Device Compatibility
PDFs can be opened on literally any device — computers, phones, tablets, and even Kindle (though the reading experience on Kindle is poor due to the small screen). Every major OS includes a PDF reader.
Key Strengths
- Exact layout preservation — what you see is what was designed
- Universal compatibility — opens on every platform
- Annotation support — highlights, notes, form filling, digital signatures
- Best for visual content — textbooks, manuals, infographics, illustrated guides
- Print-ready — PDFs are the standard for printing
Limitations
- Not reflowable — text doesn't resize, requires zooming and scrolling on small screens
- Larger file sizes — a 200-page PDF with images can easily be 20-50 MB
- Poor eReader experience — no text reflow, no Whispersync, no Word Wise
- No DRM standard — password protection exists but is easily bypassed
If you have a PDF you'd like to read on a Kindle or eReader, you can convert PDF to EPUB for a much better reading experience.
Other eBook Formats Worth Knowing
FB2 (FictionBook)
FB2 is an XML-based format popular in Russia and Eastern Europe. It separates content from formatting, making it very flexible for conversion. ConvertFiles supports FB2 to EPUB and FB2 to PDF conversions.
LIT (Microsoft Reader)
LIT was Microsoft's eBook format, now discontinued. If you have legacy LIT files, convert them to EPUB to preserve access.
AZW3 / KFX
Amazon's modern formats with enhanced typesetting. KFX is what you get when you purchase from the Kindle Store today. These are not easily converted outside of Amazon's ecosystem.
Format Comparison Table
| Feature | EPUB | MOBI/AZW | FB2 | |
|---|---|---|---|---|
| Reflowable text | Yes | Yes | No | Yes |
| Fixed layout option | EPUB 3 | No | Yes (always) | No |
| Kindle support | Via conversion | Native | Yes (limited) | Via conversion |
| Apple Books support | Native | No | Yes | No |
| Google Play Books | Native | No | Yes | No |
| Kobo support | Native | No | Yes | Yes |
| Embedded audio/video | EPUB 3 | No | No | No |
| Accessibility (ARIA) | EPUB 3 | Limited | Limited | No |
| Typical file size (novel) | 1-3 MB | 1-3 MB | 5-20 MB | 0.5-2 MB |
| Open standard | Yes (W3C) | No (Amazon) | Yes (ISO) | Yes |
| Best for | General eBooks | Kindle ecosystem | Illustrated/fixed-layout | Archival |
How to Choose the Right Format
For General Reading
Use EPUB. It's the most versatile, widely supported format. If you read on anything other than a Kindle, EPUB is the default choice. Even for Kindle users, you can convert EPUB to MOBI or send EPUBs directly to your Kindle via email.
For Kindle Users
Use AZW3 or MOBI. These formats give you the full Kindle experience including X-Ray, Word Wise, and Whispersync. If you have EPUBs, convert EPUB to MOBI before transferring to your Kindle.
For Textbooks, Manuals, and Illustrated Content
Use PDF. When page layout matters — diagrams, tables, multi-column layouts, sheet music — PDF is the only format that preserves the original design intent. Just be prepared for a suboptimal experience on small screens.
For Maximum Portability
Keep an EPUB master copy. EPUB can be converted to virtually any other format with minimal quality loss. Start with EPUB, then convert as needed for specific devices.
Converting Between eBook Formats
ConvertFiles makes it easy to convert between all major eBook formats:
- EPUB to MOBI — for reading EPUBs on Kindle
- MOBI to EPUB — to free your library from Amazon's ecosystem
- PDF to EPUB — for a reflowable reading experience
- EPUB to PDF — for printing or fixed-layout viewing
- FB2 to EPUB — for modernizing your FB2 library
- MOBI to PDF — for printing Kindle-format books
All conversions are free, require no registration, and your files are automatically deleted after processing.
Frequently Asked Questions
What is the best eBook format for Kindle? Amazon Kindle devices natively support MOBI, AZW, AZW3, and KFX formats. For the best experience with features like X-Ray and Word Wise, use AZW3 or KFX. If you have EPUB files, use ConvertFiles to convert them to MOBI before transferring to your Kindle.
Can I read EPUB files on a Kindle? Kindle does not natively support EPUB. You have two options: convert the EPUB to MOBI using a tool like ConvertFiles, or email the EPUB to your Kindle's Send-to-Kindle email address (Amazon will convert it automatically to KFX). The conversion approach gives you more control over formatting.
Is EPUB or PDF better for reading on a phone? EPUB is significantly better for phone reading. EPUB text reflows to fit your screen size, so you get a comfortable reading experience without zooming or scrolling. PDF pages are fixed-size, which means you'll need to constantly pinch-zoom and scroll horizontally on a phone — especially for A4 or letter-sized documents.
What is the difference between MOBI and AZW? MOBI is the original Mobipocket format. AZW is essentially MOBI with Amazon's DRM (Digital Rights Management) layer added. AZW3 (also called KF8) is a more modern version supporting HTML5 and CSS3. From a reading perspective, they behave similarly on Kindle devices.
Are eBook conversions lossless? Converting between reflowable formats (EPUB to MOBI, for example) generally preserves content well, though some formatting details may shift. Converting from PDF to EPUB is more lossy because the converter must infer text flow from a fixed layout — complex pages with multiple columns or embedded images may not convert perfectly. For best results, start with the source format closest to your target.
What is the FB2 format used for? FB2 (FictionBook) is an XML-based eBook format widely used in Russia and Eastern Europe. It cleanly separates content from presentation, making it excellent for conversion to other formats. If you have FB2 files, converting to EPUB is recommended for the widest device compatibility.
How do I convert a PDF textbook to an eBook format? You can convert PDF to EPUB using ConvertFiles. However, be aware that PDFs with complex layouts (multi-column text, diagrams, equations) may not convert perfectly because the reflowable format must reorganize the fixed layout. For simple, text-heavy PDFs, the conversion quality is typically very good. For heavily illustrated textbooks, keeping the PDF format is often better.
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