An eBook download often yields a directory containing multiple file extensions: .epub, .azw3, .mobi, .pdf. File conversion tools present similar arrays of options. The selection depends entirely on the target hardware.
For most devices, EPUB is the best default. For a Kindle, the right choice depends on how you deliver the book: Amazon’s Send to Kindle service accepts EPUB and converts it, while AZW3 is the dependable format for a manual USB transfer. MOBI is now mainly useful for very old devices.
The Quick Reference Table
| Format | Best For | Open Standard? | DRM Support | Reflowable? |
|---|---|---|---|---|
| EPUB | Most eReaders; Send to Kindle | Yes | Via Adobe DRM | Yes |
| AZW3 (KF8) | Kindle (2011+) | Amazon proprietary | Amazon DRM | Yes |
| MOBI | Legacy Kindles, archival compatibility | Amazon proprietary | Amazon DRM | Yes |
| Fixed-layout documents | Yes | Various | No (fixed) | |
| CBZ/CBR | Sequential art | Varies | No | No (fixed) |
EPUB — The Open Standard
EPUB (Electronic Publication) serves as the ISO-standard format for digital books, maintained by the International Digital Publishing Forum (IDPF). Version 3 (EPUB3) accommodates fundamental prose as well as complex academic texts requiring math equations, multimedia, and interactive elements.
Architecture:
An EPUB is fundamentally a ZIP archive enclosing HTML, CSS, and image assets. Renaming the .epub extension to .zip permits extraction and reveals standard web structuring. This reliance on web technologies allows for precise rendering.
Hardware and Software Compatibility:
- Kobo hardware
- Apple Books (iOS, macOS)
- Google Play Books
- Android reading software
- Desktop reading applications
- Send to Kindle (Amazon converts the file before delivery)
- Digital library infrastructure (Overdrive, Libby, Everand)
Technical Limitations:
- A Kindle does not treat an EPUB copied to its USB storage like a Kobo does. Use Send to Kindle for EPUB, or convert it to AZW3 for USB.
- Rendering of complex CSS layouts varies depending on the reading application.
Application: Default to EPUB for non-Kindle hardware and general archiving. It is also a good source file for Send to Kindle.
AZW3 (KF8) — The Modern Kindle Format
AZW3 is Amazon’s current proprietary format, introduced in 2011 alongside the Kindle Fire. Also designated as KF8 (Kindle Format 8), it is natively supported by all modern Kindle hardware.
Advantages over MOBI:
- Implementation of modern CSS capabilities (media queries, flexbox, custom typography)
- Structural table of contents via the
<nav>element - Accurate rendering of drop caps, sidebars, and advanced typographical layouts
- Enhanced image formatting (including SVG compatibility)
- Expanded file size thresholds
Application: Utilize AZW3 for Kindle hardware manufactured post-2011 when EPUB conversion is suboptimal. It yields the highest formatting fidelity within the Amazon ecosystem.
Technical Limitations: The format is closed-source and incompatible with non-Amazon hardware such as Kobo devices and Apple Books.
MOBI — The Legacy Kindle Format
MOBI served as Amazon’s primary format from the original 2007 Kindle through the Kindle Keyboard generation, built upon the acquisition of MobiPocket.
Technical Distinctions from AZW3:
- Restricted CSS compatibility
- Inefficient compression algorithms
- 4 GB theoretical file size limit
- File containers occasionally house both MOBI and AZW3 data, complicating structural integrity
Deprecation: Amazon officially discontinued MOBI support in 2022. The Send to Kindle infrastructure rejects MOBI files.
Application: Relegated exclusively to archival hardware (Kindle 1, Kindle 2, Kindle DX). Modern infrastructure requires AZW3 or EPUB.
PDF — Fixed Layout Documents
PDF isn’t really an ebook format: it’s a page format. It keeps the layout exactly as designed, which is why it can’t reflow to fit your screen. On an e-ink reader that means pinching and zooming, unless the page happens to be the size of your device.
Optimal Use Cases:
- Design-intensive publications requiring strict layout adherence
- Technical documentation housing complex diagrams
- Academic literature necessitating standardized pagination
- Print-ready materials
Structural Deficiencies for E-Readers:
- Unsuitable for continuous prose on restricted screen dimensions
- Lacks text-reflow capabilities
- Prohibits isolated typographic scaling
Note: Converting a PDF to EPUB is viable for text-based documents. Rasterized PDFs (scanned images) require Optical Character Recognition (OCR) prior to conversion.
The Format Decision Tree
Standardized deployment protocol:
Target: Amazon Kindle
├── Send to Kindle (wireless) → EPUB or supported document format
├── Manual USB transfer → AZW3
└── Very old Kindle → MOBI, only if the device requires it
Target Hardware: Kobo → EPUB
Target Software: Apple → EPUB
Content: Sequential Art → CBZ
Content: Fixed Layout → PDF
Default Standard → EPUBRendering Fidelity
Structural integrity varies significantly when processing advanced typography, such as drop caps, contextual breaks, and isolated quotes.
Kindle Rendering Hierarchy:
- AZW3: Comprehensive CSS processing; optimal structural fidelity.
- EPUB via Send to Kindle: Amazon converts it before delivery; check the result for complex books.
- MOBI: Restricted CSS processing; prone to structural degradation.
While standardized prose shows negligible variation across formats, technical documentation and design-heavy files require precise format selection to prevent rendering errors.
Format Conversion Architecture
Format conversion is viable under specific parameters:
| Source → Target | Fidelity Loss | Technical Notes |
|---|---|---|
| EPUB → AZW3 | Negligible | Near-perfect structural translation |
| EPUB → MOBI | Moderate | CSS deprecation causes minor rendering errors |
| AZW3 → EPUB | High (if DRM) | Requires unencrypted source file; DRM prevents execution |
| PDF → EPUB | Variable | Requires vector/text-based source; fails on rasterized documents |
| MOBI → AZW3 | Low | Functional upgrade; limited by source file data |
Digital Rights Management (DRM): Encrypted files prohibit structural conversion. Assets acquired via proprietary storefronts (Amazon, Apple, Adobe) remain locked to their native environments unless decrypted.
Managing formats without the busywork
You do not need to memorise every format for every book. Liber keeps the files in one local library, lets you convert in batches — free, with no limits — and gives you a clear choice when you send a book to an eReader. If you have landed here because you need one specific conversion, the step-by-step EPUB to AZW3 guide covers it. It is especially useful when a library contains a mix of EPUBs, PDFs, AZW3 files, comics, and older MOBI files.
Keep the original file whenever you can. A conversion creates a new reading copy; it is not a perfect substitute for the source, particularly when the book has complex layout or DRM.