Ebook formats are a mess. EPUB, MOBI, AZW3, KFX, PDF, KEPUB — each is slightly different, and not all of them work on every device. Here is what you actually need to know.
The Formats at a Glance
| Format | Type | Created By | Used On |
|---|---|---|---|
| EPUB | Open standard | IDPF (now W3C) | Kobo, Boox, PocketBook, apps |
| MOBI | Proprietary (legacy) | Amazon/Mobipocket | Old Kindles |
| AZW3 / KF8 | Proprietary | Amazon | Kindle (2011+, preferred) |
| KFX | Proprietary | Amazon | Kindle (2015+, current default) |
| Document | Adobe | Everything (but not great on small screens) | |
| KEPUB | EPUB variant | Kobo | Kobo only |
| CBZ/CBR | Comic archive | Community | Comics on any color device |
Kindle Formats: AZW3 vs KFX vs MOBI
Kindle has gone through three format generations:
MOBI: The original Kindle format. Still works on all Kindles, but lacks advanced typography — no embedded fonts, limited CSS, no hyphenation. Only use MOBI if you have a very old Kindle (pre-2011).
AZW3 (KF8): Kindle’s answer to EPUB2. Supports embedded fonts, better CSS, and drop caps. When you convert from EPUB to Kindle via Calibre and sideload via USB, AZW3 is what you get. It’s still the most reliable format for USB-sideloaded books.
KFX: Amazon’s current in-house format for books delivered via Wi-Fi or “Send to Kindle.” KFX supports enhanced typesetting — hyphenation, improved kerning, ligatures, and page flip. It also has the strongest DRM. If you download a book directly from Amazon’s store to a modern Kindle, you get KFX. You cannot create KFX files with free tools.
Which to use: For USB sideloading → AZW3. For cloud delivery → Send to Kindle (auto-converts to KFX). For old devices → MOBI.
EPUB: The Universal Standard
EPUB is the open standard used by everyone except Amazon. EPUB3 (the current version) supports reflowable text, embedded fonts, fixed layouts, and accessibility features like semantic tagging for screen readers.
Virtually all non-Amazon stores sell EPUB. Kobo, Boox, and PocketBook read it natively. Kindle does not — Amazon conspicuously refuses to support the format, instead converting EPUBs sent through Send to Kindle into KFX behind the scenes.
KEPUB: Kobo’s Enhanced EPUB
KEPUB is Kobo’s enhanced EPUB format. It is EPUB with extra metadata that enables reading stats (time remaining, pages per chapter), faster page turns, and better image zoom on Kobo devices.
Convert EPUB to KEPUB using Kepubify or Calibre with the KoboTouchExtended plugin. Your EPUBs will work on Kobo without conversion — KEPUB just makes them better.
PDF: Not an Ebook Format
PDF is a fixed-layout document format. It does not reflow text to fit a screen. On a 6-inch e-reader, a PDF designed for letter-size paper is basically unreadable — tiny text, constant panning and zooming.
PDFs work on 10-inch e-readers (Boox Note, Kobo Elipsa, Kindle Scribe) where the larger screen matches the original document size. For smaller devices, use K2pdfopt to reflow PDF text into a readable format, or convert PDF to EPUB in Calibre (expect formatting issues — PDF to EPUB is a lossy conversion).
Comic Formats: CBZ and CBR
CBZ and CBR are archive formats — ZIP (CBZ) or RAR (CBR) files containing JPEG/PNG pages. They’re not technically ebooks but are widely used for comics and manga.
Color E Ink devices (Kobo Libra Colour, Boox Tab Ultra C) can display CBZ files directly. Black-and-white devices technically can too, though color comics rendered in grayscale lose a lot. For converting CBZ/CBR to e-reader-friendly formats, use Kindle Comic Converter. For more, see our convert CBZ/CBR to EPUB guide.
Quick Compatibility Table
| EPUB | MOBI | AZW3 | KFX | KEPUB | ||
|---|---|---|---|---|---|---|
| Kindle | ❌ | ✅ | ✅ | ✅ | ❌ | ✅ |
| Kobo | ✅ | ❌ | ❌ | ❌ | ✅ | ✅ |
| Boox | ✅ | ✅ | ✅ | ❌ | ✅ | ✅ |
| PocketBook | ✅ | ✅ | ❌ | ❌ | ❌ | ✅ |
| reMarkable | ⚠️ | ❌ | ❌ | ❌ | ❌ | ✅ |
What You Should Actually Use
- Reading on Kindle: AZW3 (USB) or Send to Kindle (cloud)
- Reading on Kobo: KEPUB (for best experience) or EPUB
- Reading on Boox/PocketBook: EPUB
- Archiving/backup: EPUB (standard, widely supported, future-proof)
- Comics on any device: CBZ (native) or convert to EPUB/KEPUB
- Documents: PDF on 10-inch devices; convert to EPUB for smaller devices