If you've bought ebooks for more than a few years, you've ended up with a mixed library: EPUBs from Kobo, MOBI files from old Kindle purchases, AZW3 from newer Kindle buys, plus the occasional PDF for technical books. Knowing which format is which — and which to convert to which — turns a confused library into a usable one.
What each format is
- EPUB: open standard maintained by the W3C. Used by Kobo, Apple Books, Google Play Books, and most non-Kindle readers.
- MOBI: created by Mobipocket in 2000, acquired by Amazon in 2005, used as Kindle's primary format until 2011. Still widely found in older Kindle libraries.
- AZW / AZW3 / KFX: Amazon's evolving Kindle formats. AZW is the original Kindle DRM-wrapped MOBI. AZW3 is Kindle Format 8 — based on EPUB 3 with proprietary additions. KFX is the current Kindle format with stronger typography and DRM.
- PDF: not really an ebook format but often used as one. Fixed layout, doesn't reflow.
In practice, "EPUB vs MOBI" today usually means "EPUB vs Amazon's formats" — MOBI itself is mostly legacy.
Why MOBI exists at all
MOBI predates the modern ebook industry. It was designed for Palm Pilots and early e-ink devices, optimised for low memory and slow processors. When Amazon bought Mobipocket, they used MOBI as the foundation of the Kindle ecosystem.
The format has aged poorly:
- No real CSS support.
- Limited typography.
- Awkward image handling.
- No native footnotes or popup notes.
MOBI files still work on every Kindle ever made, which is why Amazon kept supporting them for decades. As of 2022, Amazon stopped letting users send MOBI files to their Kindles via email — AZW3 and EPUB are now the recommended formats.
Why EPUB won
EPUB is better technology. EPUB 3 is essentially a web page in a zip file: HTML, CSS, JavaScript, images, all packaged as a single file. The advantages:
- CSS-based typography with control over fonts, spacing, layout.
- Reflowable text that adapts to any screen size.
- Embedded fonts for guaranteed rendering.
- Audio and video support.
- Real footnotes that pop up inline.
- Accessibility through semantic HTML.
- Mathematical content via MathML.
- Open standard — anyone can read or write the format.
EPUB is also the format publishers prefer to produce. Convert once to EPUB, distribute to every retailer except Amazon (which handles the conversion automatically).
Reader support
- EPUB readers: Apple Books, Kobo, Google Play Books, Calibre, Foliate, Thorium Reader, most Android and iOS reading apps.
- MOBI readers: Kindle (all generations), Calibre, some legacy mobile apps.
- Kindles: read AZW3, KFX, and (legacy) MOBI natively. Modern Kindles also accept EPUB via send-to-Kindle.
For non-Kindle reading, EPUB is universal. For Kindle reading, send EPUB to your Kindle and Amazon converts on the fly.
Reading experience
On modern devices, EPUB usually feels better:
- Better typography with embedded fonts.
- Smoother reflow on changes to font size or screen orientation.
- Footnotes that work — tap, see the footnote inline, dismiss.
- Better navigation for tables of contents and indexes.
MOBI on a modern Kindle still works but feels older. Tables, code blocks, and footnotes are particularly rough.
File size
For an average novel:
- EPUB: 200-500 KB.
- MOBI: 300-700 KB.
- AZW3: 300-700 KB.
- PDF: 1-10 MB depending on layout and embedded fonts.
For a personal library of thousands of books, MOBI ends up taking somewhat more space than EPUB; PDF is dramatically larger.
Conversion between formats
Converting between EPUB and MOBI is nearly always lossless:
- Calibre handles every direction. The de facto tool for ebook conversion.
- Kindle Previewer (Amazon's free tool) handles EPUB → KFX/AZW3.
- Pandoc for technical conversions, especially from Markdown sources.
For DRM-free files, conversion preserves nearly everything — text, images, structure, table of contents. For DRM-protected files, removing DRM is a separate step (and the legal status varies by jurisdiction).
When to use PDF instead
PDF is the right format for ebooks when:
- Layout is essential. Comics, photo books, technical manuals with complex diagrams.
- The book has many footnotes or marginalia that need to stay positioned.
- Print-readiness matters. PDF prints predictably.
For everything else (novels, non-fiction, essays), EPUB is more readable. See PDF vs EPUB for the full comparison.
Building a personal library
A workflow that scales:
- Buy in EPUB when possible. Kobo, Google Books, Apple Books, indie publishers all sell EPUB. Amazon hides EPUB but you can email EPUBs to your Kindle.
- Strip DRM from your purchases (where legal in your jurisdiction) so you can read them on any device, in any future.
- Store in Calibre for library management, conversion, and metadata.
- Sync to your reading device of choice — Calibre integrates with Kindle, Kobo, and others.
For a Kindle-only reader, MOBI/AZW3 is fine but consider EPUB-first to keep options open.
Compatibility check
When you receive an ebook of unknown format:
- Calibre opens nearly everything. Free, cross-platform.
- EPUB opens in any modern browser if you rename to
.zipand unzip. - MOBI is harder to open without a Kindle or Calibre.
Public domain and Project Gutenberg
Project Gutenberg distributes books in multiple formats: plain text, HTML, EPUB, Kindle MOBI/AZW3. For public-domain reading, both formats are available; pick based on your device. Standard Ebooks produces beautifully formatted EPUBs of public-domain works that surpass any other available format.
Self-publishing considerations
If you're producing an ebook:
- Write in EPUB-friendly tools. Word and Pages export to EPUB. Pandoc converts from Markdown. Vellum (Mac) and Atticus (cross-platform) are dedicated ebook publishing tools.
- Test in multiple readers. EPUBs that look right in Apple Books may render differently on Kindle.
- Distribute EPUB to non-Amazon retailers. Amazon converts EPUB to KFX automatically when you upload.
- For technical content, consider PDF as a companion download — code blocks and diagrams render more reliably.
Audiobook companion
Increasingly, ebooks ship with companion audiobooks. Apple Books and Google Play Books support synchronised reading and listening. Kindle has Whispersync. EPUB 3 supports this natively (Media Overlays); MOBI doesn't.
For ebooks with audio, EPUB is the format that supports the experience.
Conclusion
EPUB is the modern standard. MOBI is legacy, kept alive mostly for older Kindle devices. Convert MOBI to EPUB for future-proofing if you're not Kindle-locked. Use PDF for layout-heavy ebooks. For PDF tasks specifically — converting, splitting, editing — Docento.app handles browser-based work without uploads. For format comparisons, see PDF vs EPUB and PDF vs HTML.