Tap an AZW3 or MOBI file on an iPhone and Apple Books won't take it. Kindle mostly won't either. Here is why, how to tell in ten seconds whether the file is locked, and which iPhone and iPad readers open the unlocked ones directly.
Case #005 — TL;DR
Apple Books reads EPUB and PDF. It does not open MOBI, AZW or AZW3, which is why tapping one in Files or Mail gets you nowhere.
Files from the Kindle Store carry Amazon DRM and open only in the Kindle app on the account that bought them. Files from Project Gutenberg, Calibre conversions and publisher downloads are usually DRM-free.
Check which one you have with the free Kindle DRM Checker. It reads the encryption flag in the file header in your browser. Nothing is uploaded.
For unlocked files, GemReader opens MOBI, AZW and AZW3 directly on iPhone and iPad. It's free, with no ads and no account, and the books sit next to your EPUBs and PDFs in one library (see our step-by-step Kindle file guide).
Verdict
Don't convert first. Check for DRM, then open an unlocked file in a reader that supports the format. A locked file belongs to the Kindle app. No converter or third-party reader can open it, and GemReader doesn't help strip DRM.
Why the iPhone won't open it
Apple Books has never supported Amazon's formats. It reads EPUB, the open standard, and PDF. A .mobi or .azw3 file in the Files app shows a blank icon because nothing installed claims it.
The Kindle app used to be the workaround, and it no longer is. Send to Kindle, Amazon's route for getting your own files into the app, stopped accepting MOBI at the end of 2023 and has never taken AZW3. Kindle on iPhone is built for books bought from Amazon.
AZW vs AZW3 vs MOBI in one minute
MOBI is the original Mobipocket format Amazon bought in 2005. Its layout options are limited, and Amazon stopped accepting it from publishers in 2021.
AZW is MOBI with Amazon's DRM wrapped around it. Nearly every .azw file came from a Kindle purchase and is locked.
AZW3, also called KF8, is the 2011 successor with proper HTML and CSS. It is what Calibre produces when you ask for Kindle output. An .azw3 file can be locked or unlocked, depending on where it came from.
KFX is Kindle-only in practice. If you have a .kfx file, it came from Amazon and it is locked.
Step 1: Check for DRM
Every MOBI-family file starts with a header that records its encryption type: 0 for none, 1 or 2 for Amazon DRM. A browser can read that field without opening the book.
Open the Kindle DRM Checker in Safari, tap to choose the file from Files or iCloud Drive, and it reports locked or unlocked. The check runs on the phone; it works with Wi-Fi off once the page has loaded. The Book Checker does the same for EPUB and PDF.
Locked? Skip to the last section. Unlocked? Any reader below will open it.
Step 2: Open the unlocked file
Install GemReader from the App Store. It runs on iPhone and iPad with iOS 17 or later. In the Files app, Mail or a Safari download, tap the share button on the file and pick GemReader. You can send up to ten files at once. Or tap Add a Book inside GemReader and choose the files from iCloud Drive or On My iPhone.
The book is encrypted into the library as it arrives. It reads with the same highlights, notes, dictionary and read-aloud as an EPUB, and every highlight becomes a card in the built-in Study deck.
Other iPhone apps that open unlocked Kindle formats: ReadEra (free, no ads or in-app purchases), Yomu (free tier, with a Pro upgrade for unlimited books and iCloud sync) and Readest (open source, with paid plans for its cloud). Each handles AZW3 well. Old MOBI files look plain in all of them, because the format itself is plain.
When converting to EPUB makes sense
Convert when you want the book in Apple Books specifically, or when an old MOBI renders badly. Otherwise opening the original avoids a step and the layout errors conversion can introduce.
The quickest route is the free MOBI to EPUB Converter, which runs in Safari on the iPhone itself: pick the file from Files, and the EPUB saves back with its chapters, images, cover and table of contents, ready to share to Apple Books. Nothing is uploaded. Calibre on a Mac or PC is the alternative for batches; the converter apps on the App Store vary widely in quality.
Conversion only works on unlocked files. Calibre stops with a DRM error on a locked one, so the error doubles as a DRM check.
It's locked. Now what?
A DRM-locked AZW or AZW3 opens in the Kindle app signed in to the buying account, and nowhere else. To read the same title in another app, check whether the publisher sells a DRM-free EPUB. Many do.
For out-of-copyright books, skip Amazon's copies. Standard Ebooks and Project Gutenberg publish clean EPUBs, and GemReader can browse and download both from inside the app.
On Android instead? The Android version of this guide covers the readers there.
A reader for the files that pass
GemReader is free, has no ads and needs no account. MOBI, AZW3, EPUB and PDF share one encrypted library on your iPhone or iPad. Highlight as you read, then study those highlights as flashcards, without exporting anything or leaving the app. Download it free and share the file to it.
Try it yourself