Ebooks come in four main formats — EPUB, MOBI, FB2, and PDF — and not every device reads each of them equally well. Let's look at what sets them apart and which one fits your reader, phone, or a print job.
EPUB — the universal standard
EPUB is an open format built around "reflowable" text: it adapts to the screen size, and you can change the font size and line spacing. It's supported by Apple Books, PocketBook, Google Play Books, and most reading apps on Android and iOS. If you're not tied to a specific device, EPUB is the right default choice.
MOBI — the format for older Kindles
MOBI is Amazon's proprietary format for early Kindle models. Current Kindles (2022 and newer) prefer AZW3 and KFX files, but they still open MOBI. Outside the Kindle ecosystem, the format is rarely supported — apps from other vendors generally can't read it.
FB2 — a format popular in Russia
FB2 (FictionBook) is common mainly among Russian-language ebook libraries. Its distinguishing trait is that the whole book is stored as a single XML file with a clear chapter structure and metadata, which makes it convenient for cataloging. The catch: outside that ecosystem, FB2 is barely supported — iPhones, iPads, and most Western reading apps can't open it directly.
PDF — a fixed layout
PDF preserves a book exactly "as is," with text positioned precisely on the page, just like a printed version. That's an advantage for books with complex layouts (comics, textbooks with formulas, illustrated albums) and for printing, but a disadvantage for reading on a small screen: the text doesn't reflow to fit the screen width, so you end up scrolling sideways constantly.
How to choose a format
- Reading on a smartphone, PocketBook, or Apple Books — EPUB.
- An older Kindle (before 2022) — MOBI or AZW3.
- Need to print the book or preserve the exact layout — PDF.
- Downloaded a book in FB2 from a Russian-language library and your reader doesn't understand it — convert it to EPUB.
How to convert a book to the format you need
If a book's format doesn't fit your device, you don't need to hunt down a different file — you can convert the one you already have. The ebook converter translates files between EPUB, MOBI, FB2, PDF, DOCX, and nearly thirty other formats using Calibre — the industry standard for working with ebooks, which preserves chapters, the table of contents, and images.
For printing or sending a book as a single document, the dedicated EPUB to PDF tool is faster than configuring the format in the general converter when you specifically need that pair of formats.
Bottom line
There's no single format for every case — the right choice depends on your device. EPUB covers most scenarios, MOBI is only needed for older Kindles, FB2 is worth converting to EPUB for compatibility outside Russian-language libraries, and PDF is chosen for its exact layout and for printing.