Ebooks

What is an EPUB file?

Updated Jul 2026

Definition

EPUB (Electronic Publication) is the standard file format for digital books on most e-readers, phones, and tablets. Its main trait is that text reflows to fit any screen size, so you can change font size without ruining the layout. Its main limitation is that Kindle devices don't read it directly, they need their own format instead.

EPUBElectronic Publication

Extension: .epub
Type: Ebooks
Typically: Most e-readers

Why EPUB exists

EPUB was built as an open standard for the publishing industry, meant to work the same way across different reading apps and devices rather than being tied to one company's hardware. That's why you'll find it used by libraries, bookstores, and independent authors alike.

Under the hood, an EPUB file is really a small bundle of web pages: HTML text, CSS for styling, and images, all zipped together with a table of contents. Your e-reader unpacks that bundle and lays the text out to fit its screen, which is why you can bump up the font size on a long train ride and the pages still look right.

People run into EPUB mostly when moving books between devices. A library loan or a purchase from one store comes as EPUB, but an older Kindle only reads its own format, so the file needs converting first. Others convert the other way, turning a PDF or a Word document into EPUB so it reflows properly on a phone instead of staying locked to one page size.

The trade-offs

Strengths

Text reflows to fit any screen size or font choice
Works across most e-readers, phones, and tablets
Keeps file sizes small since it's mostly text
Widely used by libraries and bookstores

Watch-outs

Kindle devices don't open it without converting first
Complex layouts and diagrams can reflow awkwardly
Fixed-page designs like comics translate poorly

A note on privacy

An EPUB file can carry metadata like the title, author, and publisher embedded by whoever created it, though it's far less personal than a photo's location data. Still, running it through an online converter means uploading the whole book to someone else's server. Converting on your own computer keeps the file, and any metadata in it, on your machine.

Convert an EPUB file

Questions

How do I open an EPUB file?

Most e-readers, the Books app on iPhone and Mac, and free reading apps on Windows and Android open EPUB directly. Kindle devices are the main exception, they need the file converted to a Kindle-compatible format first.

Is EPUB better than PDF for ebooks?

For reading on a phone or e-reader, yes: EPUB text reflows to fit the screen and font size you choose. PDF keeps a fixed page layout, which is better for print-like documents but awkward on smaller screens.

Why won't my Kindle open this EPUB file?

Kindle devices use their own format rather than EPUB. You'll need to convert the file to a Kindle-compatible format before it will show up on the device.

Can I convert EPUB without uploading it?

Yes. A desktop app like Morphjet converts EPUB files on your own computer, so the book's text never leaves your machine.

Morphjet opens and converts EPUB and 1,800+ other formats, all on your own computer. Launching this July.