An EPUB file is a digital book format that works on most reading devices and apps

EPUB stands for Electronic Publication. It is a file format — like .pdf or .docx — designed specifically for ebooks. When you read a book to read on a Kindle, a phone, a tablet, or a dedicated e-reader, it is often an EPUB file, though Amazon devices use their own format called MOBI or AZW instead.

The main thing that makes EPUB different from a PDF is that EPUB files reflow. That means the text adjusts to fit whatever screen you are reading on. If you make the text bigger, the page breaks change automatically. If you rotate your phone from portrait to landscape, the words rearrange themselves. A PDF, by contrast, locks the layout in place — useful for documents that need to look identical everywhere, but frustrating for reading on small screens.

EPUB files are maintained by an international standards organization called the International Digital Publishing Forum, which means the format is not owned by any single company. That is why libraries, independent publishers, and major publishers all use it. It is the closest thing to a universal ebook standard.

Key Takeaways

  • EPUB files are ebooks that automatically adjust text size and layout to fit whatever device you are reading on, unlike PDFs which stay locked in one format.
  • Most e-readers and reading apps support EPUB — including Apple Books, Google Play Books, Kobo, and most library apps — but Amazon Kindles use a different format.
  • EPUB files can include images, fonts, and interactive elements like clickable tables of contents, though the reading experience varies by device.
  • You can convert EPUB files to other formats using free software like Calibre if your preferred reading app does not support them.

How EPUB files work on different devices

When you open an EPUB file, your reading app interprets the code inside and displays it in a way that fits your screen. The same EPUB file will look slightly different on an iPhone than on a 10-inch tablet, because the app is making those adjustments in real time. This is why EPUB is popular with publishers — they create one file and it works everywhere, rather than making separate versions for each device size.

Not every app reads EPUB the same way. Some preserve the original fonts and styling more faithfully than others. Some let you adjust margins, line spacing, and background color; others do not. Apple Books, for example, gives you more control over appearance than some library apps do. This means your reading experience depends partly on which app you choose, not just on the EPUB file itself.

Where you encounter EPUB files

Libraries distribute most of their ebooks in EPUB format through apps like Libby, OverDrive, and Hoopla. If you borrow a digital book from your library, you are almost certainly getting an EPUB. Independent publishers and self-published authors often sell EPUB files directly from their websites or through retailers like Smashwords and Draft2Digital.

Major publishers also use EPUB, though they often add digital rights management — a copy-protection layer that limits how many devices you can read the book on or whether you can lend it. An EPUB file from a library might expire after three weeks; an EPUB you buy might be locked to your account. The EPUB format itself does not require this protection, but publishers add it anyway.

What is inside an EPUB file

An EPUB file is actually a compressed folder containing multiple smaller files. If you rename an EPUB to .zip and open it, you will see folders for the book's text (written in a language called XHTML), its styling instructions (CSS, the same language used for websites), and its images. There is also a table of contents file and a metadata file that contains the book title, author, and other information.

This structure is why EPUB files are so flexible. The text and styling are separate, which is what allows the text to reflow. Images can be included, but they also reflow — they shrink or grow to fit the screen. Some EPUB files include audio or video, though support for these features varies widely by reading app.

EPUB versus other ebook formats

PDF files are older and simpler. They lock the layout in place, which is why they are still used for documents, forms, and textbooks where the visual design matters. But for novels and most nonfiction, PDFs are harder to read on phones because the text does not resize.

Amazon's MOBI and AZW formats are similar to EPUB in that they reflow, but they only work on Kindle devices and the Kindle app. If you want to read on multiple devices, EPUB is more portable. Some publishers create EPUB files and then convert them to MOBI for Amazon, rather than the other way around.

Older formats like DAISY (Digital Accessible Information System) are still used for audiobooks and books for people with visual impairments, because they include accessibility features that EPUB is still catching up on. But for general reading, EPUB is the standard.

How to open and convert EPUB files

If you have an EPUB file and want to read it, the easiest path is to use an app that already supports the format. On a phone or tablet, Apple Books (iOS), Google Play Books (Android), or Libby (both) will open EPUB files directly. On a computer, you can use Apple Books, Google Play Books, or free software like Calibre.

If your preferred reading app does not support EPUB — for example, if you only have a Kindle and want to read an EPUB file — you can convert it using Calibre, which is free and open-source software. Calibre can convert EPUB to MOBI, PDF, or several other formats. The conversion is not always perfect — some formatting may be lost — but it works for most books.

You can also upload EPUB files to your Kindle using Amazon's Send to Kindle service, though Amazon converts them to their own format in the process, and the result is sometimes less polished than a native MOBI file.

Why publishers chose EPUB as a standard

Before EPUB became standard, publishers had to create separate files for each device — one for Kindle, one for iPad, one for Kobo, and so on. EPUB solved that problem by being device-agnostic. A publisher can create one EPUB file and distribute it to every retailer and library, and it will work on most reading platforms.

EPUB is also based on web standards, which means it uses the same underlying technology as websites. This makes it easier for publishers to create and update EPUB files, and easier for app developers to build readers that support them. The format has evolved over time — EPUB 3 is the current version and includes better support for images, fonts, and accessibility — but the basic principle remains the same.

Frequently Asked Questions

Can I read an EPUB file on my Kindle?

Not directly. Kindles use Amazon's proprietary format. You can convert an EPUB to MOBI or AZW using free software like Calibre, or you can email the EPUB to your Kindle using Amazon's Send to Kindle service and Amazon will convert it for you. The result usually works, but formatting may not be perfect.

Is it legal to read EPUB files from the internet?

It depends on the source. EPUB files from your library, from authors who distribute them freely, or from retailers you have purchased from are all legal. EPUB files from piracy sites are not. The format itself is legal; what matters is whether the copy you have was distributed legally.

Why does my EPUB look different on my phone than on my tablet?

EPUB files reflow to fit the screen size, so the same book will have different page breaks on a small phone screen than on a larger tablet. This is intentional — it makes the text readable on any device. If you want the book to look identical everywhere, you need a PDF instead.

Can I edit an EPUB file myself?

Yes, because an EPUB is just a compressed folder of text and styling files. You can open it with Calibre or other EPUB editors, change the text or formatting, and save it. However, if the EPUB has copy protection, you may not be able to edit it without removing that protection first, which may violate the terms of service.

What is the difference between EPUB 2 and EPUB 3?

EPUB 3 is newer and supports more features, including better image handling, embedded fonts, and accessibility tools for people with visual impairments. Most modern reading apps support both, but EPUB 3 is the current standard. If you are downloading a book today, it is almost certainly EPUB 3.