EPUB vs PDF vs MOBI vs AZW3: Complete Ebook Format Guide (2026)
If you've ever tried to put an ebook on a Kindle, you know the format alphabet soup is real. EPUB, PDF, MOBI, AZW3, KFX, KEPUB — what do they all mean, and which one should you actually use?
This guide cuts through the noise. We'll cover the four formats you'll actually encounter (EPUB, PDF, MOBI, AZW3), explain what they're good at, and tell you which to use for each ereader and use case.
The short version: if you're starting from scratch, use EPUB unless you have a specific reason not to. It's the modern open standard and works on virtually every ereader except the Kindle.
The Four Formats at a Glance
| Format | Type | Open? | Best For | Main Reader Support |
|---|---|---|---|---|
| EPUB | Reflowable | ✅ Yes (IDPF/W3C) | General reading, library loans, self-publishing | Kobo, Apple Books, Google Play Books, Nook, PocketBook, send-to-Kindle (since 2022) |
| Fixed layout | ✅ Yes (ISO) | Print-ready, technical docs, complex layouts | Universal — opens anywhere | |
| MOBI | Reflowable | ❌ Amazon proprietary (deprecated) | Don't use for new files — Amazon retired it in 2023 | Legacy Kindles (pre-2011 models) |
| AZW3 | Reflowable | ❌ Amazon proprietary (KF8) | Kindle reading with enhanced typography | Modern Kindles (Paperwhite, Oasis, Scribe) |
EPUB: The Modern Open Standard
EPUB (Electronic Publication) is the open-standard ebook format maintained by the W3C. It was first published in 2007 and has gone through three major versions — EPUB 2, EPUB 3, and the current EPUB 3.3 (released in 2023).
What Makes EPUB Special
- Reflowable: Text adapts to fit any screen size, font, and orientation
- Open standard: No single company controls it; anyone can build readers
- Rich media: Supports images, audio, video, interactive elements (EPUB 3)
- Accessibility: Built-in support for screen readers and assistive technology
- DRM support (optional): Readium LCP and Adobe DRM can be applied for library lending
- Small file size: Typically 1-3 MB for a novel
Where EPUB Works
| Device/Platform | EPUB Support |
|---|---|
| Kobo (all models since 2011) | ✅ Native |
| Apple Books (iOS, macOS) | ✅ Native |
| Google Play Books (Android, web) | ✅ Native |
| Barnes & Noble Nook | ✅ Native |
| PocketBook | ✅ Native |
| Onyx Boox | ✅ Native |
| Library lending (OverDrive, Libby) | ✅ Standard format |
| Kindle (since 2022) | ✅ Send to Kindle now accepts EPUB (the biggest change in years) |
| Kindle (pre-2022) | ❌ Required conversion to MOBI or AZW3 |
| Older Sony/Plastic Logic readers | ✅ Native |
The "Send to Kindle accepts EPUB" change in late 2022 was huge. For most people today, EPUB is the right format to start with, even if your eventual destination is a Kindle.
When to Use EPUB
- ✅ Reading novels, non-fiction, long-form articles
- ✅ Self-publishing on Apple Books, Google Play, Kobo, Barnes & Noble
- ✅ Library loans via Libby/OverDrive
- ✅ Sending to Kindle (works as of 2022)
- ✅ Any text-heavy content that should adapt to the reader's screen and preferences
When NOT to Use EPUB
- ❌ Documents that need to look exactly the same on every device (use PDF)
- ❌ Complex layouts with sidebars, multi-column text, or precise image placement (use PDF)
- ❌ Forms that need to be filled in (use PDF)
PDF: The "Looks the Same Everywhere" Format
PDF (Portable Document Format) was created by Adobe in 1993. It's now an ISO standard (ISO 32000) and is everywhere — contracts, manuals, ebooks, forms, scanned documents, academic papers.
What Makes PDF Special
- Fixed layout: What you see is what you get, on every device
- Universal support: Opens on literally any device with any app
- Print fidelity: A PDF designed for A4 paper prints exactly as expected
- Annotations and forms: Native support for fillable forms, signatures, comments
- Archival: PDF/A is a specialized variant for long-term preservation
- Larger files: Especially when fonts and high-res images are embedded
Where PDF Works
Everywhere. Phones, tablets, computers, ereaders, e-readers with reflow modes, even watches. PDF is the most universally compatible document format ever created.
When to Use PDF
- ✅ Print-ready documents (résumés, contracts, certificates, invoices)
- ✅ Documents with complex layouts (multi-column, embedded graphics, sidebars)
- ✅ Academic papers, technical manuals, datasheets
- ✅ Forms that need to be filled and signed
- ✅ Archival documents where long-term readability matters
- ✅ Scanned books (especially if you can't get an EPUB version)
When NOT to Use PDF
- ❌ Reading novels on a small ereader — PDF doesn't reflow, so you'll be zooming constantly
- ❌ Long-form content where the reader wants to control font size and spacing
- ❌ When file size matters (PDFs with embedded fonts and images can be 5-10x larger than equivalent EPUBs)
MOBI: The Deprecated Format
MOBI was the format used by early Kindles and MobiPocket Reader. Amazon officially retired MOBI support in 2023 — you can no longer email MOBI files to your Kindle, and Amazon's "Send to Kindle" service no longer accepts them.
The format is technically still supported on old Kindles (pre-2011 models) and a handful of legacy apps, but there's no reason to use MOBI for new files in 2026. Convert your old MOBI library to EPUB (which Calibre does automatically) and move on.
If You Have Old MOBI Files
Use Calibre to batch-convert them to EPUB:
- Install Calibre
- Add your MOBI files to the library
- Select all → Convert books → Output format: EPUB
- Send the resulting EPUBs to your Kindle via "Send to Kindle" or USB
AZW3: The Kindle Format (Sort Of)
AZW3 (also called KF8) is Amazon's proprietary reflowable format. It's what Amazon's own Kindle store delivers when you buy a book. AZW3 supports:
- Enhanced typography (drop caps, ligatures, kerning)
- Embedded fonts
- Better image handling
- Pop-up footnotes
- Text-to-speech
- Whispersync for syncing progress across devices
The Catch
AZW3 is proprietary. You can't officially create AZW3 files without going through Amazon's Kindle Direct Publishing (KDP) pipeline, and the format is locked to Kindle devices and apps.
The Workaround
When you send an EPUB to your Kindle via "Send to Kindle," Amazon automatically converts it to AZW3 (or its newer successor, KFX) on their server. The conversion handles typography, layout, and Whispersync setup automatically. In practice, you don't need to create AZW3 files yourself — just send EPUB to Kindle and let Amazon do the conversion.
When You Might Use AZW3 Directly
- You're sideloading via USB and want maximum compatibility with older Kindles
- You're archiving a personal library and want the exact format Amazon uses
- You're a power user who wants to fine-tune the conversion process
For 95% of users, this is unnecessary. Stick with EPUB.
Side-by-Side Comparison
| Criterion | EPUB | MOBI | AZW3 | |
|---|---|---|---|---|
| Reflowable | ✅ Yes | ❌ No (fixed) | ✅ Yes | ✅ Yes |
| Open standard | ✅ Yes (W3C) | ✅ Yes (ISO) | ❌ Proprietary | ❌ Proprietary |
| File size (typical novel) | 1-3 MB | 5-15 MB | 1-2 MB | 1-2 MB |
| Kindle native | ✅ Send-to-Kindle works (since 2022) | ✅ Yes (reflow mode on some models) | ⚠️ Old Kindles only | ✅ Yes |
| Kobo native | ✅ Yes | ✅ Yes | ❌ No | ❌ No |
| Apple Books native | ✅ Yes | ✅ Yes | ❌ No | ❌ No |
| Google Play Books | ✅ Yes | ✅ Yes | ❌ No | ❌ No |
| Library lending (Libby) | ✅ Yes | ⚠️ Limited | ❌ No | ❌ No |
| Self-publishing | ✅ Major platforms | ⚠️ Limited | ❌ Not supported | ✅ KDP only |
| Print fidelity | ❌ Not designed for print | ✅ Exact reproduction | ❌ Not for print | ❌ Not for print |
| Accessibility | ✅ Strong (EPUB 3) | ✅ Good | ⚠️ Limited | ✅ Good |
| Future-proof | ✅ Open standard, active development | ✅ Open standard, ISO | ❌ Deprecated | ⚠️ Tied to Amazon's roadmap |
Which Format Should You Use?
For Reading on a Kindle
- Best: Send an EPUB to your Kindle via Amazon's "Send to Kindle" service. Amazon converts it automatically.
- Also works: AZW3 (if you've created it via KDP or converted it yourself)
- Avoid: MOBI (deprecated)
For Reading on a Kobo
- Best: EPUB (native format)
- Also works: PDF (with reflow mode on some models)
For Reading on Apple Books
- Best: EPUB (native format)
- Also works: PDF
For Reading on a Phone or Tablet
- Best: EPUB (reflows perfectly to any screen)
- For technical docs: PDF (preserves layout)
For Library Loans (Libby, OverDrive)
- Best: EPUB (most library books are EPUB or EPUB-with-DRM)
- Also works: PDF for some titles
For Self-Publishing
- Best: EPUB (works on Apple Books, Google Play, Kobo, Nook, libraries)
- Also submit: PDF for print-on-demand
- For KDP only: AZW3 (but EPUB works there too via conversion)
For Print-Ready Documents
- Use: PDF (the only sensible choice for print)
For Complex Layouts (Manuals, Textbooks, Comics)
- Use: PDF (preserves layout exactly)
The Bottom Line
If you're starting from a Word document, Markdown file, or web page, convert to EPUB and you'll be covered for 95% of use cases. EPUB is the modern open standard, supported by every major reading platform, and Amazon's 2022 decision to accept it on Send-to-Kindle removed the last major reason to use anything else.
For free, private EPUB conversion from DOCX, HTML, or TXT, without uploading your files to a server, try converter-epub.com — runs entirely in your browser, no sign-up required.
For more advanced workflows (batch conversion, format-specific tweaking, library management), Calibre is the gold standard desktop app — free, open source, and supports every format in this guide.