Guide
Ebook formats, explained properly
Five formats cover essentially every ebook you will ever touch. Understand what each one is for – not just what it is – and every conversion decision becomes obvious.
EPUB: the standard everything else orbits
EPUB is an open standard maintained by the W3C, and under the hood it's refreshingly unmysterious: a zip archive of web pages (XHTML and CSS) plus a manifest that tells readers what's inside and in what order. That web foundation is its superpower – text reflows to any screen size, readers can restyle fonts and margins, and anything a web page can express, an EPUB can too.
Every reading system that matters supports it: Kobo, Apple Books, Google Play Books, Boox, PocketBook, every reading app worth installing – and since 2022, Amazon's own Send to Kindle. If your library standardises on one format, it's this one. Our MOBI, AZW3, FB2, DOCX and TXT converters all emit EPUB for exactly that reason.
MOBI: the retired workhorse
MOBI powered the first decade of Kindle, but it's a 2000s design: no real CSS, no embedded fonts, layout capabilities frozen in the PDA era. Amazon retired it decisively – Send to Kindle stopped accepting MOBI in 2022. The only reason to produce a MOBI today is a pre-2012 Kindle; the far more common task is rescuing old MOBI files into EPUB before the format's tooling fades away entirely.
AZW3: Kindle's native tongue
AZW3 (also called KF8) is what replaced MOBI on Kindle hardware in 2011. It's built on HTML5 and CSS – structurally a close cousin of EPUB – which is why AZW3 to EPUB conversion preserves formatting so well. You'll meet AZW3 as sideloaded files, Calibre output, and old device backups. Note the trap: books bought from the Kindle store are DRM-locked regardless of format, and no honest tool converts those.
KEPUB: EPUB wearing a Kobo badge
KEPUB isn't a separate format so much as a dialect: a standard EPUB with extra markup that unlocks Kobo's better rendering engine – the one it uses for its own store books. The payoff is faster page turns, reading statistics and sentence-accurate highlighting. If you sideload EPUBs onto a Kobo, converting them to KEPUB is the single biggest reading-experience upgrade available, and takes one drop of a file.
PDF: a printed page, not an ebook
PDF fixes ink to exact positions on a fixed page size. For print-faithful documents – textbooks, sheet music, comics – that's precisely right. For prose on a 6-inch e-reader it's precisely wrong: text can't reflow, so you're forever zooming and panning someone else's page geometry. Treat PDF as paper that happens to be digital: brilliant to print and share, miserable to read novels in.
❦
So which format should you keep?
Keep masters in EPUB. It's open, universally supported, and every other format can be produced from it. Convert on demand: to KEPUB for a Kobo, via Send to Kindle for Amazon hardware, to PDF when something needs printing. And whatever you convert, there's no reason your books should travel to a stranger's server to change costume – every tool on ebook.tools runs in your browser, so they never do.