EPUB is a free, open standard for digital books that works across devices
EPUB (Electronic Publication) is a file format for ebooks — the digital equivalent of a printed book. Unlike a PDF, which locks text and images in a fixed layout, an EPUB file lets the text reflow to fit whatever screen you're reading on, whether that's a phone, tablet, or e-reader. Most ebook retailers and libraries distribute books in EPUB format because it's free, widely supported, and not controlled by any single company.
The format has been around since 2007 and is maintained by the IDPF (International Digital Publishing Forum), now part of the W3C standards organization. When you download a book from most libraries or independent publishers, you're almost always getting an EPUB file.
Key Takeaways
- EPUB files are text-based ebooks that reflow to fit any screen size, making them different from fixed-layout PDFs.
- You can open EPUB files on phones, tablets, e-readers, and computers using free or built-in reading software.
- Some EPUB files include digital rights management (DRM) that restricts copying, while others have no restrictions.
- EPUB is an open standard not owned by any company, which is why libraries and independent publishers prefer it over proprietary formats.
How EPUB files are structured
An EPUB file is actually a compressed folder containing HTML files (the text), CSS files (styling), images, and metadata (information about the book like title and author). When you open an EPUB in a reading app, the software unpacks this folder and displays the content. This is why the same file can look different on different devices — the reading app decides how to arrange the text based on your screen size and your preferences for font, spacing, and margins.
The HTML-based structure also means EPUB files are much smaller than PDFs for the same book, since they don't store a pre-rendered image of every page. A typical novel in EPUB format might be 500 kilobytes to 2 megabytes, while the same book as a PDF could be 20 megabytes or larger.
What devices and apps can open EPUB files
Most e-readers natively support EPUB: Amazon Kindle is the major exception (it uses its own MOBI and KFX formats, though recent Kindles can read EPUB). Apple Books on iPhone, iPad, and Mac opens EPUB files directly. On Android, apps like Google Play Books, Libby, and Moon+ Reader all handle EPUB. On Windows and Mac computers, you can use free software like Calibre or Adobe Digital Editions.
Many libraries use the Libby app (formerly OverDrive), which downloads EPUB files to your device and lets you read them with a built-in reader. If you're reading on a Kobo e-reader, Nook, or most other dedicated ebook devices, EPUB is the default format. The widespread support is one reason EPUB became the standard — you're not locked into one company's ecosystem the way you are with Kindle books.
DRM and whether you can copy text from an EPUB
Some EPUB files include DRM (digital rights management), which is encryption that prevents copying, printing, or sharing the file. Library books almost always have DRM, which is why you can't copy a paragraph from a library ebook and paste it elsewhere. Books you buy from independent publishers or download from Project Gutenberg typically have no DRM, so you can copy, print, and share them freely.
Whether an EPUB has DRM depends on the publisher or retailer, not the format itself. You can tell whether a file is protected by trying to copy text from it — if nothing happens, it likely has DRM. Some reading apps show a lock icon next to the title to indicate protection.
EPUB versus PDF and other ebook formats
A PDF is a fixed layout — the text and images stay in the same position no matter what device opens it. This works well for documents with precise formatting (like textbooks with diagrams or forms), but it's frustrating for novels because you have to pinch and zoom to read on a small screen. EPUB reflows the text to your screen, so you never have to zoom.
Amazon's proprietary formats (MOBI and KFX) work similarly to EPUB but only on Kindle devices and apps. Kindle books are often cheaper than EPUB versions because Amazon subsidizes the price, but you can't read them on other devices. MOBI is older and less capable than modern EPUB; KFX is newer but still exclusive to Amazon. If you want maximum flexibility and device independence, EPUB is the better choice.
How to convert files to and from EPUB
If you have a book in another format and want to convert it to EPUB, the free software Calibre handles most conversions: PDF to EPUB, MOBI to EPUB, Word documents to EPUB, and many others. You import the file, select EPUB as the output format, and click convert. The quality of the conversion depends on how complex the original file is — a simple novel converts cleanly, but a textbook with heavy formatting may need manual fixes.
You can also convert EPUB to other formats using Calibre, though converting to PDF loses the reflow advantage (the text becomes fixed again). Some online converters exist, but Calibre is more reliable and keeps your files on your own computer rather than uploading them to a server.
Where to find EPUB books
Your local library almost certainly lends EPUB ebooks through apps like Libby, OverDrive, or Hoopla — you just need a library card. Project Gutenberg offers over 70,000 free EPUB books in the public domain. Independent publishers and many self-published authors distribute EPUB directly from their websites. Larger retailers like Apple Books, Google Play Books, and Kobo sell EPUB ebooks, though some may include DRM.
Amazon is the main retailer that doesn't use EPUB — it sells Kindle books in its own format. If you want to read a Kindle book on a non-Amazon device, you'd need to convert it, which is legally complicated because Kindle books have DRM that's illegal to remove in most countries.
Frequently Asked Questions
Can I read an EPUB file on my Kindle?
Recent Kindle devices and the Kindle app can read EPUB files, but older Kindles cannot. The Kindle app on phones and tablets accepts EPUB, but the files won't sync across your Amazon library the way native Kindle books do. For the best Kindle experience, convert EPUB to Kindle format using Calibre first.
What's the difference between EPUB2 and EPUB3?
EPUB3 is the newer standard (released in 2011) and supports more features like embedded audio, video, and better typography. EPUB2 is older but still widely used. Most reading apps support both, so you don't usually need to worry about which version you have. Newer books tend to be EPUB3, while older library books may be EPUB2.
Can I edit an EPUB file myself?
Yes, because EPUB is just a compressed folder of HTML and CSS files. You can extract it with any unzip tool, edit the HTML in a text editor, and repackage it. For simpler edits like changing fonts or margins, most reading apps let you adjust these settings without touching the file itself. Calibre also has an EPUB editor for more complex changes.
Why does my library book stop working after two weeks?
Library EPUB files include DRM that expires on the checkout date. When the loan period ends, the file becomes unreadable — this is how libraries enforce lending limits without physical books. You can renew the loan through your library app before the deadline, or check it out again after it expires.
Is EPUB the same as a website?
EPUB uses the same underlying technology as websites (HTML and CSS), but it's a self-contained file rather than pages served from a server. This means an EPUB works offline and doesn't change — a website can update its content anytime. Some EPUB files include interactive elements like buttons and forms, but they're designed for reading, not browsing.