File Format Cheat Sheet for Authors

EPUB, MOBI, AZW3, PDF: every retailer seems to want something slightly different, and most of the confusion is outdated information. Here's the current, accurate picture, plus where to convert safely if you need to.

Randall Wood 3 min read
File Format Cheat Sheet for Authors
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File Format Cheat Sheet for Authors

Few things generate more low-grade confusion for new indie authors than ebook file formats, partly because a lot of advice circulating online is several years out of date. The short version: the format landscape has actually simplified significantly, and most authors only ever need to think about two formats, EPUB for ebooks and PDF for print. Here's the current, accurate picture, format by format.

The Format Landscape, Plain Language

Field / Spec

Value / Requirement

Notes

EPUB

The universal ebook standard. Reflowable, meaning text automatically resizes and rewraps to fit whatever screen it's displayed on, from a phone to a dedicated e-reader. This is what every major retailer wants, Amazon's KDP included, regardless of what older advice may say about Amazon requiring something else.


MOBI

Amazon's original Kindle format, now fully discontinued. Amazon stopped accepting MOBI uploads for new KDP titles years ago, and MOBI is no longer relevant for anything you're actively publishing today. If older advice tells you to prepare a MOBI file for Amazon, that advice is outdated; ignore it.


AZW3 (also called KF8)

Amazon's current internal Kindle format. You do not create or upload this yourself. When you submit an EPUB to KDP, Amazon automatically converts it to AZW3 behind the scenes for delivery to Kindle devices. Knowing this name mainly helps when the term comes up in author discussions, not because you'll ever directly handle the file.


PDF

A fixed-layout format, meaning the page looks identical no matter what device displays it, which is exactly why it's the right choice for print interiors and exactly wrong for ebooks. PDFs don't reflow to fit a phone or e-reader screen, making them a poor reading experience as an ebook, but the correct, expected format for KDP Print, IngramSpark, and other print-on-demand submissions.


What This Actually Means for Your Submissions

  • Ebook submissions, to Amazon KDP, Apple Books, Kobo, Google Play, and essentially every other retailer covered throughout this resource library, should be EPUB. This is the single format to focus on producing well; every platform either accepts it directly or, in Amazon's case, converts it automatically on their end

  • Print submissions, to KDP Print, IngramSpark, or any other print-on-demand service covered in this resource library's production sections, should be PDF, formatted specifically for the fixed trim size and bleed requirements that print demands

  • You do not need to produce MOBI, AZW3, or KFX files yourself under any normal circumstances. If a tool or service is asking you for one of these, double-check that you're actually following current submission requirements rather than outdated instructions

  • Formatting tools referenced throughout this resource library, including Vellum and Atticus, generate a properly structured EPUB and a print-ready PDF as their standard output, meaning most authors never need to think about file format conversion manually at all

If You Ever Do Need to Convert

Occasionally a real need to convert between formats does come up, an old MOBI file from years ago that needs updating, or a one-off need to view an EPUB in a different format. Calibre is the long-standing, free, genuinely capable tool for this, handling conversion between virtually every ebook format along with broader library management, metadata editing, and cover handling. For a quick, one-off conversion without installing software, a range of free online converters handle the same basic format-to-format conversion, similar in spirit to general utility sites like tinywow.com, though for anything beyond a simple one-off conversion, Calibre's free desktop tool gives more reliable, controllable results.

⚠ Be cautious converting a fixed-layout PDF into a reflowable format like EPUB. The conversion can produce messy, unpredictable results, since PDF was never designed to reflow, and any conversion attempting to force it into a reflowable format requires real manual cleanup afterward. This conversion direction, PDF to EPUB, is meaningfully less reliable than the reverse.

A Quick Reference by Platform

Field / Spec

Value / Requirement

Notes

Amazon KDP (ebook)

EPUB (Amazon converts internally to AZW3/KFX for Kindle delivery)


Amazon KDP Print / IngramSpark

PDF, formatted to the specific trim size and bleed requirements


Apple Books, Kobo, Google Play, Barnes & Noble, most independent retailers

EPUB


Library lending platforms (OverDrive, Libby, hoopla)

EPUB



Conclusion

The format landscape that once genuinely confused authors has settled into something simple: EPUB for ebooks, PDF for print, and nothing else to actively think about for most submissions. If you ever encounter outdated advice insisting on MOBI, treat it as a sign the source hasn't been updated recently. For the rare actual conversion need, Calibre remains the reliable free standard, with simple online converters available for quick one-off jobs.

- Randall



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