An AZW3 or MOBI file either opens in any decent Android reader or opens in nothing but Kindle — and the difference is a single flag in the file header. Here is how to tell which one you have in ten seconds, which readers open the free ones, and what to do about the locked ones.
Case #003 — TL;DR
MOBI, AZW and AZW3 are Amazon's ebook formats. Files from Project Gutenberg, Calibre conversions, publisher direct downloads and most email attachments are DRM-free and open in third-party Android readers.
Files that came from the Kindle Store or the Kindle app's own download folder carry Amazon DRM and only open in Kindle apps on the buying account. No third-party reader can open them.
Tell them apart in your browser: drop the file into the free Kindle DRM Checker. It reads the encryption flag in the file header; nothing is uploaded.
For unlocked files, GemReader opens MOBI, AZW and AZW3 directly on Android — free, no ads, no account — alongside EPUB and PDF in the same library.
Verdict
If the file is DRM-free, install a reader that supports the format and open it — no conversion needed. If it is DRM-locked, the honest answer is the Kindle app; conversion tools cannot touch it, and stripping DRM is a legal grey area GemReader doesn't help with. Check first, then act.
AZW vs AZW3 vs MOBI — what you're holding
MOBI is the original Mobipocket format Amazon bought in 2005. It is limited: no real CSS, no embedded fonts, tables that fall apart. Amazon stopped accepting new MOBI uploads from publishers in 2021.
AZW is MOBI with Amazon's DRM wrapper. Almost every .azw file in the wild came from a Kindle and is locked.
AZW3 (also called KF8) is the 2011 successor: proper HTML5 and CSS3, embedded fonts, fixed layouts. It renders far better and is the format Calibre produces when you ask for a Kindle output. An .azw3 file can be locked or unlocked; where it came from decides which.
KFX is newer still and Kindle-only in practice. Third-party readers don't open it; if you have a .kfx file, it came from Amazon and it is locked.
Step 1: Check for DRM before installing anything
Every MOBI-family file starts with a PalmDOC header that includes an encryption-type field. 0 means no encryption; 1 or 2 means Amazon DRM. It sits at a fixed offset, which is why a browser can read it without opening the whole book.
The Kindle DRM Checker does exactly that. Choose the file, and it reports locked or unlocked without sending anything anywhere — you can turn Wi-Fi off after the page loads and it still works. The broader Book Checker does the same for EPUB and PDF if you have a mixed folder.
If the file is locked, stop here and read the last section. If it is unlocked, any reader below will open it.
Step 2: Open the unlocked file
GemReader opens MOBI, AZW and AZW3 natively on Android 10 and up. Import from the file picker, a folder, or by sharing the file from another app; it is encrypted into the library on arrival and reads with the same highlights, notes, dictionary and read-aloud as an EPUB. Highlights you make go straight to the built-in flashcard Review page.
Other Android readers that open unlocked Kindle formats: Moon+ Reader (free with ads, or Pro), ReadEra, Librera and KOReader. All handle AZW3 well; older MOBI files render acceptably in each but with the format's own limitations.
Kindle for Android opens them too, of course — sideload by copying the file into the Kindle folder or using Send to Kindle. That is the right choice if the rest of your library is Kindle purchases and you want one app.
When to convert to EPUB instead
Usually never. If your reader opens AZW3 directly, conversion adds a step and can introduce layout errors. Convert when the file is old MOBI and looks broken, or when you need to open the book on a device that only speaks EPUB.
Calibre on a desktop is the reliable route: add the book, convert to EPUB, send the result to your phone. On Android alone, there is no conversion tool worth recommending for this; a desktop round-trip takes two minutes and gets it right.
Conversion only works on unlocked files. Calibre stops with a 'locked by DRM' error otherwise — which is itself a valid DRM check if you already have Calibre installed.
It's locked. Now what?
A DRM-locked AZW or AZW3 opens in Kindle apps signed in to the account that bought it, and nowhere else. If you want to read a Kindle purchase in a different app, check whether the publisher sells a DRM-free EPUB directly — many do, and some retailers (Kobo, Google Play Books) sell the same title with lighter or no DRM.
For out-of-copyright titles, skip Amazon's copies entirely: Standard Ebooks and Project Gutenberg publish clean EPUBs with no DRM, and GemReader can browse and download both from inside the app.
A reader for the files that pass
GemReader is free, has no ads and needs no account. MOBI, AZW3, EPUB and PDF sit in one encrypted library; highlight as you read, then study those highlights as flashcards without exporting anything or leaving the app. Download for Android and import the file.
Try it yourself
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