EPUB vs MOBI vs PDF vs AZW3 — eBook Format Comparison Guide
You have an eBook file and it won't open on your device. Or you're about to publish your first book and every platform wants a different format. Or someone sent you a .mobi file and you have no idea what to do with it. The eBook format landscape is genuinely confusing — there are at least six common formats, each tied to specific devices and ecosystems. This guide breaks down every major eBook format, explains which devices support which, and tells you exactly which format to use for any situation.
EPUB — The Universal Standard
EPUB (Electronic Publication) is the open standard for eBooks, maintained by the W3C. Think of it as the MP3 of eBooks — not owned by any single company, supported almost everywhere. EPUB files are essentially zipped packages of HTML, CSS, and metadata, which means they handle reflowable text beautifully. The text adapts to your screen size, font preferences, and reading settings. Make the font bigger, and the text reflows to fit. Switch from portrait to landscape, and the layout adjusts.
Supported by: Apple Books (iPhone, iPad, Mac), Google Play Books, Kobo readers, Barnes & Noble Nook, most Android reading apps (Lithium, Moon+ Reader, ReadEra), Calibre on desktop, and virtually every non-Amazon e-reader. Notably, Amazon Kindle now supports EPUB as of late 2022 via the Send to Kindle feature, though native Kindle Store purchases still use Amazon's proprietary formats.
MOBI and AZW3 — Amazon's Kindle Formats
MOBI was the original Kindle format, developed by Mobipocket (which Amazon acquired in 2005). It works, but it's old — limited formatting options, no support for modern CSS, and relatively large file sizes. Amazon replaced it with AZW3 (also called KF8 or Kindle Format 8), which supports richer formatting, embedded fonts, HTML5, CSS3, and better image handling. If you see a .azw3 file, it's essentially Amazon's enhanced version of MOBI.
In practice, Amazon has been moving away from MOBI entirely. Kindle devices and apps now use AZW3 for anything purchased from the Kindle Store, and Amazon stopped accepting MOBI uploads for new Kindle Direct Publishing titles in 2023. If you're creating eBooks for Kindle readers, you're better off using EPUB (which Amazon converts on their end) or going through KDP's conversion pipeline. If you have old MOBI files you want to read on non-Kindle devices, convert them to EPUB.
PDF — Universal but Inflexible
PDF is the format everyone knows, and it opens on every device ever made. But PDF was designed for fixed-layout documents — the text stays exactly where it was placed, at the exact size it was set. On a desktop monitor, a PDF textbook looks great. On a phone screen, that same PDF requires constant pinching, zooming, and horizontal scrolling. For long-form reading, PDFs are miserable on anything smaller than a tablet.
That said, PDFs are the right choice for certain types of eBooks: textbooks with complex layouts, technical manuals with diagrams, cookbooks with precise photo placement, and any publication where the visual layout is integral to the content. If your eBook is mostly text (novels, non-fiction, essays), EPUB is almost always better. If it's heavily visual with specific layout requirements, PDF might be the way to go. You can always convert your PDF to other formats if needed.
FB2 — The Format You've Never Heard Of
FB2 (FictionBook) is a structured XML-based eBook format that's hugely popular in Russia and Eastern Europe but barely known elsewhere. It stores text with semantic markup — chapters, quotes, epigraphs, and poems are tagged as such, not just styled visually. This makes FB2 files excellent for automated formatting and metadata extraction. If you're distributing eBooks to Russian-speaking audiences, FB2 support is essential. Several Russian eBook stores (LitRes, for example) distribute in FB2 format. Most major reading apps like Moon+ Reader and FBReader support it natively.
Reflowable vs Fixed Layout — Why It Matters
This is the single most important concept in eBook formats. Reflowable formats (EPUB, MOBI, AZW3) let the text adapt to the reader's screen and preferences. The reader controls font size, line spacing, and margins. The content flows like water into whatever container it's given. Fixed layout formats (PDF, and some special EPUB variants) lock the content into a specific page size and arrangement. What you see is exactly what the author designed — no font changes, no reflow.
For novels and text-heavy books, reflowable is always better. Readers want to adjust the font size and reading comfort. For children's picture books, graphic novels, art books, and technical manuals with precise diagrams, fixed layout preserves the intended design. Most eBook converters handle reflowable content well, but converting a fixed-layout PDF to reflowable EPUB requires careful attention — the conversion may produce messy results that need manual cleanup.
DRM and Format Lock-In
DRM (Digital Rights Management) is the copy-protection technology that prevents you from freely copying and sharing purchased eBooks. Amazon uses its own DRM on Kindle books, Apple uses FairPlay DRM on Apple Books purchases, and Adobe uses Adobe DRM on many EPUB books. A DRM-protected book can only be read on authorized devices, and you can't convert it to another format. DRM-free books, on the other hand, can be freely converted between formats and read on any compatible device.
When you buy an eBook, check whether it's DRM-free. Many independent publishers and platforms (like Smashwords, many Kobo titles, and books purchased directly from authors) sell DRM-free EPUBs that you can convert and read anywhere. If you're self-publishing, seriously consider releasing DRM-free — readers strongly prefer the flexibility, and DRM has never been shown to prevent piracy.
Quick Format Cheat Sheet
Kindle readers: AZW3 (purchased) or EPUB (sideloaded via Send to Kindle).
Apple Books: EPUB.
Kobo: EPUB or KEPUB.
Google Play Books: EPUB or PDF.
Nook: EPUB.
Universal fallback: PDF (but no reflowable text).
Russian readers: FB2 or EPUB.
Self-publishing upload: EPUB (works on all platforms including Amazon KDP).
Which Format Should You Choose?
If you're a reader trying to open a file: check what device you're using and convert to the format it supports. Kindle users with EPUB files can send them directly to their Kindle via Amazon's Send to Kindle service. Non-Kindle users with MOBI files should convert to EPUB. If you're publishing: create your book in EPUB format. Every major platform accepts EPUB, including Amazon KDP (which converts it internally). EPUB is the future-proof choice, and maintaining a single source format is far easier than juggling MOBI, AZW3, and PDF versions.
