The ebook format landscape is a product of Amazon's decision to build a walled garden. EPUB is an open W3C standard supported by Apple Books, Google Play Books, Kobo, Nook, Calibre, and essentially every reading app and device on the market — except Kindle. Amazon built their own stack: MOBI, then AZW3 (KF8), then KFX. The result is two ecosystems that work great internally but require conversion to cross between.
Understanding what each format actually is clears up most of the confusion. EPUB is a ZIP archive containing XHTML files, CSS stylesheets, and metadata — it's basically a website in a container. MOBI is a legacy format based on PalmDOC from the 2000s. AZW3 is Amazon's modern format, which is essentially EPUB3 wrapped in Amazon's DRM container. And PDF is a fixed-layout document format that was never designed for reading on variable-sized screens.
This guide covers what each format does, where it works, and when to convert between them. If you're publishing, distributing, or just trying to get a book onto your device, here's what you need to know.
EPUB: The Open Standard That Won
EPUB (Electronic Publication) is maintained by the W3C and is the closest thing to a universal ebook format. It's supported by Apple Books, Google Play Books, Kobo, Nook, Calibre, Aldiko, Lithium, Moon+ Reader, and dozens more. The only major holdout is Kindle — and even that changed in 2022 when Amazon started accepting EPUB via Send-to-Kindle.
What's Inside an EPUB
Rename any .epub file to .zip and extract it. Inside you'll find:
META-INF/container.xml— points to the root content filecontent.opf— the package document listing all chapters, images, and metadata (title, author, language, ISBN)toc.ncxornav.xhtml— the table of contents- XHTML files — one per chapter, styled with CSS
- Images, fonts, and other assets embedded in the archive
This HTML/CSS foundation means EPUB text reflows to fit any screen. On a phone, you might see 30 words per line. On a tablet, 60. The reader app controls font size, typeface, margins, and line spacing. This reflowability is what makes EPUB fundamentally superior to PDF for reading on variable-sized screens.
EPUB2 vs EPUB3
EPUB2 (2007) supports basic XHTML 1.1 and CSS2. It handles text, images, and simple formatting well. Most ebooks you'll encounter are still EPUB2 — it covers 90%+ of use cases for novels and non-fiction.
EPUB3 (2011, revised 2023) adds HTML5, CSS3, JavaScript, MathML, SVG, embedded audio/video, and fixed-layout support. Fixed-layout EPUB3 is used for children's books, cookbooks, and comics where page design matters as much as content. EPUB3 also supports vertical writing systems (CJK text), ruby annotations, and bidirectional text.
In practice, most reading apps support the EPUB3 features that matter (better CSS, embedded fonts, SVG) and ignore the ones that don't (JavaScript in ebooks never caught on). If you're creating ebooks, author in EPUB3 — backward compatibility with EPUB2 readers is generally fine for text-heavy books.
Amazon's Kindle Formats: MOBI, AZW3, and KFX
Amazon has used three distinct ebook formats over the years, each building on the last. Understanding the differences matters because Kindle devices and apps handle them differently.
MOBI: The Legacy Format
MOBI is based on the Mobipocket format Amazon acquired in 2005. It uses PalmDOC compression and a limited subset of HTML for content. MOBI supports basic formatting — bold, italic, headings, images — but lacks modern typographic features like embedded fonts, drop caps, or complex CSS layouts.
Amazon stopped accepting MOBI uploads to the Kindle Store in 2021. Older Kindle devices (pre-2012) and the Kindle app on very old OS versions still read MOBI, but there's no reason to create new MOBI files today. If someone sends you a .mobi file, convert it to EPUB for a better reading experience on non-Kindle devices.
AZW3 (KF8): The Modern Kindle Format
AZW3, also called Kindle Format 8 (KF8), launched with the Kindle Fire in 2011. It's essentially an EPUB3-like format using HTML5 and CSS3, wrapped in Amazon's container with optional DRM. AZW3 supports embedded fonts, SVG, fixed layout, and the same styling capabilities as EPUB3.
This is the format you should target for Kindle. Calibre converts EPUB to AZW3 cleanly because the underlying content model is nearly identical. When Amazon's Kindle Previewer ingests an EPUB, it's essentially creating an AZW3 internally. Convert EPUB to AZW3 here.
KFX: Amazon's Newest Format
KFX is Amazon's proprietary enhanced format used for books purchased from the Kindle Store since ~2015. It adds improved typesetting (better hyphenation, kerning, and justification), Bookerly/Ember font rendering, X-Ray content integration, and Page Flip. KFX files use Amazon's JXR image format and their proprietary container.
You can't create KFX files directly — Amazon generates them from your uploaded EPUB or DOCX during the publishing process. KFX only works on Kindle devices and apps. It's not a format you need to worry about for personal use or distribution outside the Kindle Store.
Send-to-Kindle Now Accepts EPUB
In April 2022, Amazon began accepting EPUB files via Send-to-Kindle (email and app). This was a watershed moment. Previously, getting a non-Amazon ebook onto a Kindle required converting to MOBI or AZW3. Now you can email an EPUB directly to your Kindle's @kindle.com address, and Amazon converts it server-side.
Amazon simultaneously deprecated MOBI for Send-to-Kindle — .mobi files sent after August 2022 no longer appear on Kindle devices. The practical upshot: EPUB is now the right format for everything, including Kindle delivery. Convert other formats to EPUB, send to Kindle, and Amazon handles the rest.
PDF as an Ebook: When It Works (and When It Doesn't)
PDF (Portable Document Format) preserves exact page layout — fonts, spacing, margins, graphics placement. That's its strength and its fatal flaw for ebook use. A PDF rendered on a 6-inch Kindle screen shows tiny, unreadable text because the pages were designed for 8.5x11" or A4 paper. Pinch-to-zoom isn't reading; it's suffering.
When PDF Is the Right Choice
- Textbooks with complex layouts — Multi-column pages, sidebars, equations mixed with diagrams. Reflowing this to EPUB destroys the spatial relationships between elements
- Sheet music — Musical notation requires precise positioning that reflowable text can't handle
- Technical manuals with schematics — Wiring diagrams, architectural drawings, and annotated images need fixed positioning
- Print-replica editions — When the physical page design IS the product (art books, magazines, graphic novels)
- Academic papers — Two-column layouts with footnotes, citations, and figures. Most academic publishing still revolves around PDF
When PDF Is the Wrong Choice
If the book is primarily flowing text — novels, biographies, self-help, business books — PDF is strictly worse than EPUB. The reader can't adjust font size, line spacing, or margins. Dark mode doesn't work properly (you get a white page on a dark background). Text-to-speech stumbles on PDF column layouts. Bookmarks and highlighting are clunkier than in reflowable formats.
Converting PDF to EPUB (try it here) is one of the hardest format conversions to get right. The converter must infer document structure — which text is a heading, which is body copy, which is a caption — from position and font size alone. Multi-column layouts often come out as interleaved garbage. Scanned PDFs (images of text) require OCR first. If you have the source document (Word, InDesign, LaTeX), export to EPUB directly instead of going through PDF.
Less Common Ebook Formats
FB2 (FictionBook)
FB2 is an XML-based ebook format that's extremely popular in Russia and Eastern Europe but barely known in the West. It stores book content in a well-structured XML document with explicit tags for titles, epigraphs, poems, citations, and body text — more semantic markup than EPUB typically uses.
FB2's structured metadata makes it excellent for library management. Russian ebook libraries like Flibusta standardized on it. If you receive an FB2 file, converting to EPUB is straightforward since both formats use semantic markup for content. Calibre handles FB2 natively.
CBR/CBZ: Comic Book Archives
CBR and CBZ aren't really "ebook" formats — they're renamed RAR and ZIP archives containing sequentially numbered image files (usually JPG or PNG). Comic readers (CDisplayEx, YACReader, Panels on iOS) open the archive, sort the images alphabetically, and display them page-by-page.
CBR uses RAR compression. CBZ uses ZIP. Since ZIP is more universally supported and RAR is proprietary, CBZ is generally preferred for new comic archives. There's no text layer, no metadata standard (though ComicInfo.xml is a common convention), and no reflowable content — these are literally folders of page images in an archive. Convert CBR to CBZ if you need to switch from RAR to ZIP-based archives.
Dead Formats: LRF, LIT, PDB
LRF (Sony Reader format) was used by Sony's e-readers from 2006-2010. Sony abandoned it in favor of EPUB. LIT (Microsoft Reader) was Microsoft's ebook format from 2000-2012. Microsoft shut down the Reader app and the format died with it. PDB (Palm Database) was used by Palm OS devices. If you have files in any of these formats, convert to EPUB immediately — the ecosystems that supported them no longer exist, and finding working readers gets harder every year.
Device Compatibility Matrix
| Format | Kindle | Apple Books | Google Play Books | Kobo | Nook | Calibre | Generic E-ink |
|---|---|---|---|---|---|---|---|
| EPUB | Via Send-to-Kindle | Full | Full | Full | Full | Full | Full |
| MOBI | Legacy only | No | No | No | No | Full | Varies |
| AZW3 | Full | No | No | No | No | Full | No |
| KFX | Full | No | No | No | No | Read only | No |
| Basic | Full | Full | Full | Full | Full | Full | |
| FB2 | No | No | No | Full | No | Full | Via apps |
| CBR/CBZ | No | No | No | Full | No | Full | Via apps |
The practical takeaway: EPUB works everywhere except Kindle (and even Kindle accepts it now via email). AZW3 is Kindle-only. PDF works everywhere but reads poorly on small screens. If you have one format and need another, convert — the content is the same, only the container changes.
Conversion Quality: What to Expect
Not all ebook conversions are equal. Some are lossless container swaps. Others are fundamentally lossy because the formats model content differently.
Conversions That Work Well
- EPUB to MOBI/AZW3 — Clean conversion. Both use HTML/CSS internally. EPUB to MOBI and EPUB to AZW3 preserve formatting, images, and metadata reliably. Calibre is the gold standard tool
- MOBI to EPUB — Generally good. MOBI's simpler formatting maps cleanly to EPUB. Convert MOBI to EPUB here
- FB2 to EPUB — FB2's semantic markup converts well to EPUB's HTML structure. Try it here
- EPUB to PDF — Good, but you lose reflowability. The converter renders each page at a fixed size. Convert EPUB to PDF
- Any reflowable format to TXT — Strip all formatting, keep the text. Useful for text analysis or reading in minimal apps. EPUB to TXT
Conversions That Are Fundamentally Hard
- PDF to EPUB — The hardest common conversion. PDF has no concept of "paragraphs" or "chapters" — it's positioned glyphs on a page. The converter must reconstruct document structure from visual layout. Multi-column PDFs, PDFs with headers/footers, and scanned PDFs all produce poor results. Try it, but set expectations accordingly
- Scanned PDF to anything — A scanned PDF is just images. Converting requires OCR (optical character recognition) first, which introduces errors. Accuracy depends on scan quality, font, and language
- CBR/CBZ to reflowable formats — Comic archives are images. Converting to EPUB embeds those images in an EPUB container but doesn't make the text reflowable. You get a fixed-layout EPUB that's functionally identical to the original
DRM and Metadata
DRM: The Conversion Blocker
DRM (Digital Rights Management) is the reason you can't simply convert a Kindle Store purchase to EPUB. Amazon wraps AZW3/KFX files in their DRM scheme, which encrypts the content and ties it to your Amazon account. Apple Books uses FairPlay DRM on purchased books. Adobe Digital Editions uses ADEPT DRM for many library ebooks.
No legitimate conversion tool will strip DRM. Calibre explicitly refuses to handle DRM-protected files. If you try to convert a DRM-protected AZW file, the conversion will fail or produce garbage. The legal implications of DRM removal vary by jurisdiction, but the practical reality is: DRM-protected files cannot be converted between ecosystems. This is by design — it's the lock-in mechanism that format-specific stores depend on.
DRM-free ebooks (from publishers like Tor, O'Reilly, Smashwords, and most indie authors) convert freely between any formats.
Why Metadata Matters
Ebook metadata — title, author, series name, cover image, language, publisher, ISBN — is what library management software (Calibre, Apple Books, Kobo's library) uses to organize and display your books. A file without metadata shows up as "Unknown" by "Unknown" with no cover art.
EPUB stores metadata in the OPF file using Dublin Core standards. MOBI/AZW3 use a proprietary metadata format. When converting between formats, metadata usually transfers cleanly. The most common loss is the cover image — always verify the cover survived conversion, especially for MOBI to EPUB. Calibre's metadata editor lets you fix or add metadata to any supported format.
Practical Recommendations
- Publishing an ebook? Author in EPUB3. Upload EPUB to Apple Books, Google Play, Kobo, and Smashwords directly. Upload EPUB or DOCX to Amazon KDP (they convert to KFX internally). One source format, all platforms
- Reading on Kindle? If the file is EPUB, email it to your @kindle.com address. If it's MOBI, convert to EPUB first, then send via email. If it's AZW3, it already works
- Reading on anything else? EPUB. If the file is MOBI or AZW3, convert to EPUB or convert AZW3 to EPUB
- Archiving ebooks? Keep the EPUB. It's an open standard backed by the W3C. MOBI and AZW3 depend on Amazon's continued support. In 20 years, EPUB will still be readable. MOBI might not be
- Reading comics? CBZ for storage and compatibility. Most comic readers handle CBZ natively. Convert CBR to CBZ for better compatibility
The ebook format war is effectively over. EPUB won the open ecosystem. Amazon maintained their proprietary stack but accepted EPUB as an input format. For publishers, authors, and readers, the practical answer is simple: EPUB for everything, with conversion to AZW3 when you specifically need native Kindle support.
If you have ebooks stuck in legacy formats, convert them now while the tools exist. MOBI to EPUB, AZW3 to EPUB, FB2 to EPUB — all produce clean results. The content is what matters; the container is just plumbing.