Ebook File Formats Explained: EPUB, MOBI, AZW3, FB2, PDF, TXT
Download a few ebooks and you will quickly run into a confusing alphabet soup of file extensions: EPUB, MOBI, AZW3, FB2, PDF, TXT. Knowing the difference between these ebook file formats saves you from “this file won’t open” headaches and helps you pick the right book version to download. This guide explains each of the six common formats — where it came from, what it is good at, where it falls short, and which apps open it.
A Quick Mental Model
Before the details, here is the big idea. Ebook formats split into two camps:
- Reflowable formats (EPUB, MOBI, AZW3, FB2, TXT) store structured text that rearranges to fit your screen and font size. Great for reading novels on a phone.
- Fixed-layout formats (PDF) freeze the page exactly as designed. Great when the layout itself is the content.
If you only remember one thing from the epub vs mobi vs azw3 debate, it is this: they are all reflowable text formats with different histories, and a modern reader app can usually open all of them.
EPUB — The Open Standard
Origin: Created by the International Digital Publishing Forum, EPUB is the closest thing to a universal ebook standard.
Pros: Open, widely supported, reflowable, supports rich formatting, images, and accessibility features. The default for public-domain libraries and most non-Kindle stores.
Cons: Not natively supported by older Kindle hardware (though that is changing). DRM-locked EPUBs from some stores still need their own app.
Opened by: Nearly every reader app except, historically, Kindle. See what an EPUB file actually is for what is inside one, how to read EPUB on Android for the practical steps, or the EPUB reader page.
MOBI — The Old Kindle Format
Origin: Based on the Mobipocket format, MOBI became an early Amazon Kindle standard.
Pros: Reflowable and widely compatible with older readers and conversion tools.
Cons: Largely deprecated. Amazon itself has moved away from MOBI in favor of newer formats, and it handles modern formatting and styling poorly compared to EPUB. You will still encounter MOBI files in older collections.
Opened by: Many third-party readers, including Aurora Reader, which can open DRM-free MOBI files on Android. Follow how to read MOBI files on Android for import steps and troubleshooting.
AZW3 — The Newer Kindle Format
Origin: Amazon’s successor to MOBI (also called KF8), designed to support richer formatting like better fonts, styling, and layout.
Pros: More capable than MOBI, with improved typography and structure.
Cons: It is an Amazon format, so DRM-protected AZW3 files bought from the Kindle Store are locked to Amazon’s ecosystem. DRM-free AZW3 files, however, open in compatible third-party readers.
Opened by: Kindle apps and several third-party readers, including Aurora Reader, which can open DRM-free AZW3 files. How to open AZW3 on Android covers the practical steps; if you have both formats of the same book, AZW3 vs MOBI compares them properly.
FB2 — The Structured XML Format
Origin: FictionBook (FB2) originated in the Russian-speaking ebook community and is built on clean XML.
Pros: The FB2 file format stores well-structured metadata and text, making it lightweight and easy to convert. Popular for fiction and large free libraries.
Cons: Less common in Western mainstream stores, so support varies by app.
Opened by: Dedicated readers that support FB2, including Aurora Reader. If Android says it cannot open one, how to open an FB2 file on Android walks through it, and the FB2 reader page covers the format support itself.
PDF — The Fixed-Layout Workhorse
Origin: Created by Adobe, PDF preserves a page exactly as designed.
Pros: Perfect fidelity for textbooks, forms, comics, and anything where layout matters. Universal — opens almost everywhere.
Cons: Fixed layout means awkward zooming on phones, limited font control, and weaker text-to-speech support. For a deeper look, read EPUB vs PDF.
Opened by: Practically every device and app, including Aurora Reader. See the PDF reader page or follow how to read PDF ebooks on Android for phone-specific setup.
TXT — Plain and Simple
Origin: The humble plain-text file, as old as computing itself.
Pros: Universally compatible, tiny file size, completely future-proof. Nothing to break.
Cons: No formatting, images, chapters, or styling — just raw text. Fine for simple documents and some classic public-domain works.
Opened by: Everything, including Aurora Reader, which applies your chosen fonts and themes to make plain text comfortable to read. The TXT reader page covers format support, while how to read TXT like an ebook includes encoding fixes.
Format Comparison Table
| Format | Type | Origin | Best for | DRM concern? |
|---|---|---|---|---|
| EPUB | Reflowable | Open standard (IDPF) | Most ebooks, novels | Sometimes (store-locked) |
| MOBI | Reflowable | Mobipocket / early Kindle | Older Kindle libraries | Sometimes |
| AZW3 | Reflowable | Amazon (KF8) | Newer Kindle files | Sometimes |
| FB2 | Reflowable | FictionBook (XML) | Fiction, free libraries | Rarely |
| Fixed | Adobe | Textbooks, forms, comics | Rarely | |
| TXT | Plain text | Universal | Simple documents | No |
A Word About Converting Formats
If a book is in a format your reader does not like, you can often convert it. Free desktop tools such as Calibre convert between EPUB, MOBI, AZW3, FB2, and more in a few clicks. Two honest caveats:
- Conversion cannot remove DRM. A store-locked file stays locked no matter what you convert it to. Converting only works on DRM-free books.
- Some formatting may shift during conversion, especially with complex layouts.
In most cases, the simplest path is not converting at all — it is using a reader that already opens the format you have.
One App, Six Formats
Rather than juggling separate apps for each extension, Aurora Reader opens all six of these formats — EPUB, PDF, FB2, MOBI, AZW3, and TXT — in a single library, with 10 themes, 7 fonts, and full customization. Import via the file picker or “Open with” from any app, and built-in SHA-256 duplicate detection keeps your shelf tidy.
Curious where to find free books to fill that library? Start with where to find free ebooks legally, then read what is OPDS for browsing those catalogs from inside the app. New to reading on a phone? How to read ebooks on Android covers the whole setup, or explore more guides on the blog. Aurora Reader is free, with no ads — download it here.