Download a book and you often end up with a folder full of extensions: .epub, .azw3, .mobi, .pdf. Converters offer the same alphabet soup. Which one you want comes down to a single thing: the device you are going to read on.
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) is the standard format for digital books, an ISO standard that started at the International Digital Publishing Forum. The current version, EPUB 3, handles anything from a novel to an academic text with equations, video and interactive exercises.
How it is built
An EPUB is a ZIP file with HTML, CSS and images inside. Rename it to .zip, unpack it, and you will find a small website in there. That is why it renders so consistently across very different devices: underneath, it is ordinary web technology.
Where it opens:
- Kobo, where it looks best
- Apple Books
- Google Play Books
- Reading apps on Android, such as Moon+ Reader or ReadEra
- Desktop apps, such as Calibre or Thorium Reader
- Send to Kindle, which converts it before delivery
- Library lending (Overdrive, Libby, Everand)
Where it falls short:
- A Kindle does not treat an EPUB copied over USB the way a Kobo does. For EPUB, use Send to Kindle. If you are going by cable, convert it to AZW3 first.
- Complex layouts look different depending on the app that opens them.
When to use it: by default on anything that is not a Kindle, and for archiving. It is also the best source file for Send to Kindle.
AZW3 (KF8): the modern Kindle format
AZW3 is Amazon’s current format. It arrived in 2011 and is also called KF8 (Kindle Format 8). Any Kindle from then on opens it without converting anything.
What it gains over MOBI:
- Modern CSS: media queries, flexbox and embedded typography
- A real table of contents, through the
<nav>element - Drop caps, sidenotes and complex layouts that look the way the publisher designed them
- High-resolution images and vector graphics (SVG)
- Much larger files without trouble
When to use it: on any Kindle from 2011 on, when you cannot send an EPUB. It is what preserves the design best inside Amazon’s ecosystem.
Where it falls short: it is closed. A Kobo or Apple Books will not open it.
MOBI: the old Kindle format
MOBI is the format of the original 2007 Kindle. It comes from MobiPocket, the company Amazon bought in 2005, and it carried the whole ecosystem until AZW3 arrived.
Where it falls short of AZW3:
- Basic CSS, with no complex positioning and no media queries
- Worse compression, so files take up more room
- A theoretical 4 GB ceiling per file
- Sometimes one file holds both MOBI and AZW3 data, and you cannot always tell which one the device is opening
Where it stands today: Amazon dropped it in 2022, and Send to Kindle has rejected MOBI files since February 2023.
When to use it: only if you have a Kindle 1, 2 or DX. On anything modern it adds nothing.
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.
What it is actually good for:
- Magazines and comics, where the design is the content
- Technical manuals with diagrams
- Academic texts where the page has to stay the same so it can be cited
- Anything you are going to print
Where it falls short:
- On a small screen it reads badly, because the page cannot adapt
- You cannot raise the type size without pushing half the page out of view
One note: you can pull the text out of a PDF and rebuild it as an EPUB, and some tools do it well. The more real text the PDF holds, the better it comes out. If the pages are scans, what you have is an image, and it needs OCR before any conversion.
The Format Decision Tree
If you would rather decide at a glance:
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 → EPUBHow faithful the book looks
The format also decides how closely the book matches what its publisher designed. Drop caps, ornamented section breaks and pull quotes only survive in the modern formats.
On a Kindle, best to worst:
- AZW3: full CSS support and the most faithful layout.
- EPUB via Send to Kindle: Amazon converts it before delivery, so check the result on complex books.
- MOBI: limited CSS support, and layouts often break.
An ordinary novel looks much the same in any format. A book with design to it, a manual or an illustrated one, only holds up in EPUB or AZW3.
What you lose when you convert
Converting almost always works, but not every conversion costs the same:
| Source → Target | Fidelity Loss | What to expect |
|---|---|---|
| EPUB → AZW3 | Negligible | The CSS carries over almost untouched |
| EPUB → MOBI | Moderate | Modern CSS is stripped back on purpose |
| AZW3 → EPUB | High (if DRM) | Needs an unencrypted source file. DRM blocks it |
| PDF → EPUB | Variable | Needs a text-based source. Fails on scanned pages |
| MOBI → AZW3 | Low | Only the wrapper changes, nothing else is lost |
If the book has DRM, there is no conversion. A file encrypted by Amazon, Apple or Adobe cannot be converted, because the lock sits inside the file itself.
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 and 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.