Apple Books is already on your iPhone, and for a lot of people it's enough. This guide is for when it isn't: Kindle-format files, comics, a no-ads library, cross-device sync beyond iCloud, or highlights you want to study. Six free apps, what each does best and where each falls short. Written by the makers of one of them, with that bias declared up front.
Case #007 — TL;DR
Already installed and good: Apple Books. EPUB and PDF, iCloud sync, polished. No MOBI or AZW3, and no comics.
Most formats, free, no ads, no purchases: ReadEra. Opens nearly everything, including Kindle formats, DjVu and Word files. No sync between devices.
Sync everywhere: Readest. Open source, runs on every platform, syncs through its own cloud or yours. Paid plans for cloud storage and extras.
Studying, with highlights that become flashcards, saved workbook answers and an encrypted library: GemReader. Free with no paid tier, no ads and no telemetry. No cloud sync. We make it.
Verdict
Stay on Apple Books if your books are EPUBs and iCloud covers your devices. Switch to ReadEra for format breadth without paying, Readest to keep reading position in step across iPhone, Android and desktop, PocketBook for Adobe-DRM store books, and GemReader if you read to learn and want highlights to become a study deck without exporting anything.
A note on who is writing this
GemReader is our app, which makes a 'best of' list from us suspect. So the rule here is that every app gets its strongest case and its real weaknesses, ours included, and the picks are by use case, not a ranking. Prices are US App Store prices as listed in November 2026.
Apple Books: the default that's hard to beat
Apple Books renders EPUB 2 and 3 properly, fixed-layout books included, and opens PDFs. With iCloud for Books turned on, your position, highlights and imported PDFs follow you across iPhone, iPad and Mac. It is free, already installed and has no ads.
The limits are formats and control. It does not open MOBI, AZW3 or comic archives. Highlights stay inside the app unless you share them out by hand, and there is no review of them. Organising a large collection of your own files is clumsy next to the dedicated readers below.
ReadEra: the most formats for nothing
ReadEra's iOS app has no ads and no in-app purchases. It opens EPUB, PDF, MOBI, AZW3, FB2, DjVu, DOC and DOCX, RTF, TXT, CBZ and CBR, ODT and CHM. Highlights and notes are free, and text-to-speech reads aloud with a choice of voices.
What it lacks is sync: progress and highlights stay on the device you made them on. It has no spaced repetition. If you read on one device and want every file to just open, ReadEra is an excellent default.
Yomu: clean, with Kindle formats
Yomu opens EPUB, MOBI, AZW, AZW3, CBZ, CBR and PDF in a calm, well-designed reader. Highlights and notes export as text, Markdown or JSON, which suits anyone feeding a notes app.
The free tier is a trial in practice: it limits how many documents you keep and can show ads. Yomu Pro ($7.99, or $12.99 as a 'pay more' option) removes the limit and adds an iCloud library synced across your Apple devices. It needs iOS 17 or later.
Readest: open source, synced everywhere
Readest is open source and runs on iPhone, iPad, Mac, Windows, Linux, Android and the web. It reads EPUB, PDF, MOBI, AZW3, FB2, CBZ, TXT and Markdown, has text-to-speech with downloadable offline voices, and syncs through Readest Cloud or your own iCloud, Google Drive, OneDrive, WebDAV or S3. It even syncs progress with KOReader.
The core reader is free. Paid Plus and Pro plans ($4.99 and $9.99) and storage add-ons cover the hosted cloud and extras. The interface is busier than Yomu's. It's the pick for keeping your reading position in step across everything you own.
PocketBook Reader: the one that opens Adobe DRM
PocketBook's free app reads a long list of formats, from EPUB, FB2, MOBI and PDF to DjVu, DOCX, RTF and comics, and offers PDF reflow for small screens. Its distinguishing feature is Adobe DRM: EPUBs and PDFs bought from bookstores that use Adobe's protection open here when they won't open in most other readers.
The interface carries the feel of PocketBook's e-ink devices, and the settings run deep. Choose it if your library includes Adobe-protected purchases.
GemReader: for people who read to learn
GemReader is built around the read-mark-review loop. Highlight in one of six colours, type a note, and it is a card in a built-in spaced-repetition Study tab, with no export and no second app. Existing Anki decks (.apkg, .txt) import into the same queue, which spares you AnkiMobile's $24.99. Interactive EPUB workbooks save your answers automatically, and PDFs highlight and turn into cards like any EPUB.
It opens EPUB, PDF, MOBI, AZW, AZW3, FB2, TXT and CBZ in one library. Every book is AES-256 encrypted on import, and PIN-locked folders hide sensitive reading. There is no account, no ads, no paid tier and no analytics of any kind. Free books from Project Gutenberg and Standard Ebooks download in-app, there's a built-in dictionary, and read-aloud works on any chapter. On iPad it has its own tablet layout, not a stretched phone screen.
The weaknesses: no cloud sync (backups go to a folder or Google Drive, encrypted, instead of a continuous sync); no DjVu, CBR or Word files; fewer display settings than PocketBook or Readest; iOS 17 or later; and the iPhone app is new, released in November 2026. Not sure a file will open? The free Book Checker reports DRM and damage in Safari without uploading anything.
Quick picks by situation
Kindle purchases with DRM: none of these. Only the Kindle app opens them. Check a file with the Kindle DRM Checker before assuming otherwise; DRM-free MOBI and AZW3 open in ReadEra, Yomu, Readest and GemReader (how to open them).
Library loans: Libby, your library's own app. Textbooks and lecture PDFs before an exam: GemReader. Comics: ReadEra or Yomu for .cbr, and any of the four above for .cbz. No ads and nothing to buy: ReadEra or GemReader. Reading position synced across iPhone, Android and desktop: Readest.
Studying from highlights? The full comparison of export routes is in How to Turn Ebook Highlights into Flashcards on iPhone. On Android? See the Android version of this list.
Try the one built for studying
If your reading comes with an exam attached, GemReader is the one on this list made for it. Textbooks, lecture PDFs and workbooks in one library on iPhone and iPad. Highlight as you read, then study those highlights as flashcards, without exporting anything or leaving the app. Free, no ads, no account. Download it free.
Try it yourself