An EPUB file is a digital book format that adjusts to fit whatever screen you're reading it on

An EPUB (short for Electronic Publication) is a standardized file format for ebooks — digital books you read on phones, tablets, e-readers, or computers. Unlike a PDF, which locks text and images in a fixed layout, an EPUB file reflows its content to match your screen size and your reading preferences. If you increase the font size, the text wraps to fit. If you rotate your device, the layout adjusts automatically.

The file itself is actually a compressed folder containing HTML files (the text), CSS files (the styling), images, and metadata (the book's title, author, publication date). Your reading app unpacks this folder and displays it as a book. This is why EPUB files are so much smaller than printed books — they contain only the words and images, not a fixed page layout.

Most major publishers and independent authors distribute ebooks in EPUB format because it works across devices. Amazon uses its own format called MOBI for Kindle devices, but nearly every other ebook platform — Apple Books, Google Play Books, Kobo, most library systems — uses EPUB or a variant of it.

Key Takeaways

  • EPUB files are ebooks that automatically adjust their text size and layout to match whatever device you're reading on, unlike PDFs which stay fixed.
  • You need an EPUB reader app to open these files — common options include Apple Books, Google Play Books, Kobo, Calibre, and many library apps.
  • EPUB files are much smaller than PDFs or printed books because they contain only text and images without a locked page layout.
  • Most ebooks sold outside of Amazon use the EPUB format, making it the standard for independent publishers and library systems.

How EPUB files differ from PDFs and other ebook formats

A PDF is a snapshot — it preserves the exact appearance of a printed page, with fixed margins, page breaks, and font sizes. This makes PDFs reliable for documents that need to look identical everywhere, but it makes them difficult to read on small screens because you have to zoom and scroll. An EPUB file, by contrast, is more like a set of instructions: it tells your reading app what the text says and how it should be styled, but lets the app decide how to display it based on your screen and preferences.

Amazon's Kindle format (MOBI and its newer variant KF8) works similarly to EPUB — it reflows text and adjusts to screen size — but it is proprietary to Amazon. You can read MOBI files on Kindle devices and the Kindle app, but not on most other platforms without converting them first. EPUB is an open standard maintained by the International Digital Publishing Forum, which means any developer can build an app that reads EPUB files without paying Amazon or anyone else.

Other formats exist for specific purposes: DAISY files are designed for people with visual impairments and include audio narration; AZW3 is Amazon's current Kindle format; and some publishers use HTML or plain text files. But EPUB remains the most common format for general-audience ebooks outside the Kindle ecosystem.

Which apps and devices can open EPUB files

Most devices have at least one built-in or free app that reads EPUB files. On iPhone and iPad, Apple Books (formerly iBooks) opens EPUB files natively and syncs your reading progress across devices. On Android phones and tablets, Google Play Books reads EPUB files, as does the free Kobo app. On Windows and Mac computers, you can use Calibre (a free, open-source ebook manager), Apple Books on Mac, or web-based readers like Thorium Reader.

Dedicated e-readers like Kobo and most non-Amazon devices support EPUB directly. Many public library apps — including Libby, OverDrive, and Hoopla — distribute ebooks in EPUB format and include their own reading apps. If you read an EPUB file from a library or publisher, you straightforward open it with whichever app you prefer, and it displays the book with your chosen font size, background color, and line spacing.

The main exception is Amazon's Kindle ecosystem. Kindle devices and the Kindle app do not natively support EPUB files. If you want to read an EPUB on a Kindle, you must convert it first using a tool like Calibre, which can transform EPUB files into the MOBI or AZW3 formats that Kindle understands.

Why publishers and libraries use EPUB format

EPUB became the standard for ebook distribution because it solves a real problem: readers have different devices, screen sizes, and accessibility needs. A single EPUB file works on an iPhone, an Android tablet, a Kobo e-reader, and a computer — without the publisher having to create separate versions for each platform. This saves time and money for publishers and makes it easier for readers to move between devices without losing their library.

Libraries particularly rely on EPUB because it is not locked to a single company's ecosystem. A library can lend the same EPUB file to patrons using different devices, and each patron's reading app handles the display. This flexibility is why most public library ebook systems use EPUB rather than Kindle format.

EPUB also supports features that matter for serious readers: bookmarks, annotations, adjustable fonts, and built-in dictionaries. Publishers can embed fonts, images, and even basic interactivity into EPUB files, though most trade books keep it straightforward — just text and cover art.

DRM protection and copying EPUB files

Many EPUB files sold by publishers come with DRM (Digital Rights Management) protection, which is encryption that prevents you from copying or sharing the file. When you buy an ebook from Apple Books or most other retailers, the file is locked to your account. You can read it on any device where you sign in with that account, but you cannot give the file to someone else or read it with an app that is not authorized by the retailer.

Library EPUB files also use DRM, but with an expiration date — the book automatically becomes unreadable after the loan period ends, without you having to return it. Some independent publishers and authors distribute EPUB files without DRM, which means you can copy and share them freely (though you should still respect the author's copyright).

If you own an EPUB file and want to remove DRM protection for personal use, tools like Calibre can help, though the legality varies by country. In the United States, the Digital Millennium Copyright Act makes circumventing DRM illegal even for content you own, though enforcement against individuals is rare.

How to get and organize EPUB files

You can obtain EPUB files from several sources. Public libraries distribute them through apps like Libby and OverDrive — you borrow them the same way you borrow physical books, and they return automatically. Online retailers like Apple Books, Google Play Books, and Kobo sell EPUB files. Project Gutenberg and Standard Ebooks offer free EPUB downloads of public-domain books. Many independent authors sell EPUB files directly from their websites or through platforms like Smashwords.

To organize a personal library of EPUB files, Calibre is the standard tool. It runs on Windows, Mac, and Linux, and lets you import EPUB files, edit metadata (title, author, cover art), convert between formats, and sync books to your e-reader. You can also straightforward store EPUB files in a folder on your computer or cloud storage and open them with your preferred reading app whenever you want to read.

If you buy EPUB files from a retailer, they are usually stored in your account's cloud library rather than as files on your device. You do not need to manage them manually — your reading app downloads them when you want to read and keeps them synced across your devices.

Frequently Asked Questions

Can I read an EPUB file on my Kindle?

Not directly. Kindle devices and the Kindle app only read Amazon's proprietary formats (MOBI and AZW3). You can convert an EPUB file to MOBI or AZW3 using free software like Calibre, then transfer the converted file to your Kindle. Some Kindle devices also accept email delivery of converted files.

What is the difference between EPUB2 and EPUB3?

EPUB3 is the newer standard and supports more advanced features like audio, video, and interactive elements. Most ebooks still use EPUB2, which is simpler and works on older devices. Reading apps handle both formats automatically, so you do not need to worry about the difference — your app will display either version correctly.

Can I edit an EPUB file myself?

Yes, because an EPUB file is a compressed folder of HTML and CSS files. You can unzip it, edit the HTML in a text editor, and rezip it. Calibre also includes an editor for making changes to EPUB files. However, editing is mainly useful if you are self-publishing or working with your own files — editing a retailer's EPUB will break the DRM protection.

Why is my EPUB file so small compared to the printed book?

An EPUB contains only the text and images, not the paper, binding, or air between pages. A 300-page novel might be 500 kilobytes as an EPUB but weigh several hundred grams in print. The file size depends on how many images the book contains — a novel with no images is tiny, while a graphic novel or illustrated book is much larger.

Can I read EPUB files offline?

Yes, once you read an EPUB file to your device, you can read it without an internet connection. Library apps and retailer apps may require occasional online verification to confirm your loan or purchase is still valid, but the actual reading happens offline.