EPUB vs MOBI vs PDF vs AZW3: Which Ebook Format Should You Use?
A practical comparison of the four formats you'll actually run into.
If you've ever tried to move a book onto an e-reader, you've hit the format question. EPUB? MOBI? AZW3? PDF? They all hold a book, but they behave very differently — and picking the wrong one leads to broken formatting, files that won't open, or text you can't resize. This guide breaks down each format in plain language and tells you which one to reach for in 2026.
The short answer
For almost everyone, the answer is EPUB. It's an open standard, it reflows to fit any screen, and since late 2022 even Amazon Kindle supports it natively. MOBI is effectively retired, AZW3 is Kindle-only, and PDF is a fixed-layout format that fights small screens. If you're producing a book today, make an EPUB — and only convert to something else if a specific device forces you to.
EPUB — the open standard
EPUB (Electronic Publication) is the closest thing the industry has to a universal ebook format. It's essentially a zipped bundle of XHTML, CSS, and images, which means the text reflows: readers can change the font, size, margins, and line spacing, and the book adapts. That's the single most important property of a good ebook.
- Works on: Kobo, Apple Books, Google Play Books, Nook, Thorium, Calibre, and Kindle (2022+).
- Reflowable: yes — text adapts to any screen.
- Best for: novels, non-fiction, notes, documentation — anything text-heavy.
Because it's an open standard maintained by the W3C, EPUB isn't tied to any single company. A file you make today will still open in a decade, on hardware that doesn't exist yet.
MOBI — the retired format
MOBI was Amazon's original Kindle format (inherited from Mobipocket). For years it was the file you needed to email to your Kindle. That era is over: Amazon stopped accepting MOBI through Send to Kindle in 2022 and now recommends EPUB instead. MOBI has no support for modern typography and is best avoided unless you're maintaining a very old Kindle library.
- Works on: older Kindle devices and apps only.
- Reflowable: yes, but with dated rendering.
- Best for: nothing new — treat it as legacy.
AZW3 — Kindle's own format
AZW3 (also called KF8) is Amazon's proprietary successor to MOBI. It supports richer formatting than MOBI and is what many modern Kindles use internally. The catch is in the name: it's proprietary. You can't reliably open an AZW3 on a Kobo or in Apple Books. Since Kindle now ingests EPUB directly and converts it on Amazon's side, there's rarely a reason to create AZW3 yourself.
- Works on: Kindle devices and apps only.
- Reflowable: yes.
- Best for: advanced Kindle-only workflows via Kindle Previewer.
PDF — great for print, bad for reading on small screens
PDF is a fixed-layout format. Every page is frozen at a specific size, like a photograph of a printed page. That's perfect for forms, invoices, and anything that must print identically everywhere — but it's painful on a 6-inch e-reader, where you end up pinching, zooming, and scrolling sideways to read a single paragraph.
- Works on: essentially everything.
- Reflowable: no — layout is fixed.
- Best for: print-ready documents, worksheets, anything with a precise layout.
Sending a PDF to a Kindle "because it's easy" almost always disappoints. The text won't resize, footnotes break, and margins eat the screen. For reading, convert to EPUB instead.
Side-by-side comparison
| Format | Open standard | Reflows | Kindle | Kobo / Apple / Google |
|---|---|---|---|---|
| EPUB | Yes | Yes | Yes (2022+) | Yes |
| MOBI | No | Yes | Legacy only | No |
| AZW3 | No | Yes | Yes | No |
| Yes | No | Poorly | Poorly |
So which should you use?
- Reading on any e-reader: EPUB.
- Publishing or sharing a book: EPUB.
- Print-exact documents: PDF.
- Legacy Kindle library: keep MOBI/AZW3, but make new files as EPUB.
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