What Is an EPUB File? The Ebook Format Explained

How EPUB works under the hood

An EPUB file is, much like the modern Office formats, a ZIP archive — inside it are the book's chapters as HTML files, its styling as CSS, its images, fonts, and a manifest describing the reading order and table of contents. In other words, an ebook is essentially a small, self-contained website packaged into one file. This is why the text can reflow: it is web content, and the reading app lays it out dynamically just as a browser lays out a web page.

Because it is built on open web technology, EPUB supports adjustable font sizes and typefaces, night mode, bookmarks, embedded fonts and images, and — importantly — accessibility features that let screen readers navigate the book by chapter and heading.

EPUB vs PDF for reading

The core difference is fixed versus reflowable layout. A PDF preserves an exact page design, which is essential for anything where the layout is the point — a magazine, a form, a heavily designed book, a document meant to print identically. But on a small screen a PDF forces you to zoom and pan around a page that was designed for paper, which is a poor reading experience.

An EPUB, by reflowing, adapts to the screen and reader preferences, making it far more comfortable for long-form reading, especially novels and other text-driven books. The trade-off is that it does not guarantee a fixed page appearance — the same book looks different depending on the device and the reader's font settings, which is exactly what makes it comfortable but unsuitable when precise layout matters.

Where EPUB is supported

EPUB is supported by most ebook ecosystems: Apple Books, Google Play Books, Kobo, the Nook, and countless reading apps on phones and tablets all open EPUB directly. The notable exception has historically been Amazon's Kindle, which favoured its own formats — though Kindle now accepts EPUB by converting it on import. On a computer, free apps and browser extensions open EPUB, though it is not something a plain web browser or office suite typically displays without help.

For publishing, EPUB is the standard deliverable for most ebook stores, which is why authors and publishers export to it. If you are distributing a text-driven book, EPUB reaches the widest range of readers in the most comfortable form.

How to open and work with an EPUB

To read an EPUB, use an ebook app — Apple Books on Apple devices, Google Play Books, Kobo, or one of many free readers on any platform. OfficePad focuses on documents, PDFs, images and developer tools rather than ebook conversion, so for turning a document into an EPUB you would use dedicated ebook software; but if your goal is simply a fixed, shareable, printable version of a document instead of a reflowable ebook, exporting to PDF from a word processor is the straightforward route, and OfficePad can help with the PDF and document side of that work.

Frequently asked questions