What Is an EPUB File? EPUB vs PDF and How to Open One

An EPUB file (.epub) is an e-book in the open EPUB standard. EPUB stands for electronic publication. The file is a ZIP archive that holds the book's text as XHTML pages, CSS styles, images and a package file with the title, author and reading order. Most EPUBs are reflowable: the text rewraps to fit any screen and font size.

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That is the short answer to what is an EPUB file. Below you'll see who maintains the format, what is inside the ZIP, how EPUB compares with PDF, how to open an EPUB on each device, and how to convert, create or edit one.

What is an EPUB file? The format in brief

EPUB is the most widely supported open e-book format: Apple Books, Google Play Books, Kobo e-readers and free desktop apps such as Calibre and Thorium Reader all read it. Because an EPUB is built from web technology (HTML, CSS and SVG), a book behaves much like a small offline website that a reading app displays one screen at a time.

Who maintains EPUB

EPUB was developed by the International Digital Publishing Forum (IDPF), which approved EPUB 3.0 in October 2011. On 30 January 2017 the IDPF combined with the World Wide Web Consortium (W3C), which now maintains the standard. EPUB 3.3 became a W3C Recommendation on 25 May 2023, together with EPUB Reading Systems 3.3 and EPUB Accessibility 1.1, and W3C published a revised edition in January 2026. EPUB 3.3 is backward compatible: any valid EPUB 3.2 book is also valid EPUB 3.3.

What is inside an EPUB file

Rename book.epub to book.zip and any archive tool will open it. A minimal EPUB 3 book looks like this:

sample-voyage.epub
├── mimetype                 "application/epub+zip"
├── META-INF/
│   └── container.xml        points to the package file
└── OEBPS/
    ├── content.opf          package: metadata, manifest, spine
    ├── nav.xhtml            navigation document (table of contents)
    ├── style.css
    ├── chapter1.xhtml
    └── chapter2.xhtml

Each part has a job:

The package document for this example is short:

<package xmlns="http://www.idpf.org/2007/opf" version="3.0" unique-identifier="uid">
  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/">
    <dc:identifier id="uid">urn:uuid:9f1c2b4e-3d5a-4c7b-8e2f-1a6d0c9b7e21</dc:identifier>
    <dc:title>The Sample Voyage</dc:title>
    <dc:creator>Ada Example</dc:creator>
    <dc:language>en</dc:language>
  </metadata>
  <manifest>
    <item id="nav" href="nav.xhtml" media-type="application/xhtml+xml" properties="nav"/>
    <item id="ch1" href="chapter1.xhtml" media-type="application/xhtml+xml"/>
    <item id="ch2" href="chapter2.xhtml" media-type="application/xhtml+xml"/>
  </manifest>
  <spine>
    <itemref idref="ch1"/>
    <itemref idref="ch2"/>
  </spine>
</package>

(A real book also lists its CSS and images in the manifest and adds a dcterms:modified date.)

Reflowable vs fixed-layout EPUB

EPUB supports two layouts, set in the package document:

EPUB 3 can also carry audio, video, MathML formulas and media overlays, which synchronize narration with the highlighted text. Support for these extras varies between reading apps.

EPUB and DRM

EPUB itself is open, but a store or library can lock a copy with DRM (digital rights management):

A DRM-protected EPUB opens only in apps authorized for that DRM, and converters can't read its encrypted chapters. Books you write yourself and public-domain titles are usually DRM-free.

EPUB vs PDF: which format to use

EPUB vs PDF comes down to one question: should the text adapt to the screen, or should the page look exactly the same everywhere?

EPUB PDF
Layout Reflowable text (fixed layout optional) Fixed pages with exact positions
On a phone Text rewraps; reader chooses font and size You zoom and scroll around the page
File size Compressed ZIP of text, CSS and images; usually small for text-heavy books Stores every page's layout and usually embeds fonts; often larger for the same text
Accessibility HTML structure, reader-controlled text; EPUB Accessibility 1.1 is part of the standard Good only when the PDF is tagged; fixed pages make large text hard to read on small screens
Editing HTML and CSS inside; edit with Sigil or Calibre Needs a PDF editor; text changes are awkward
Printing Not designed for print; depends on the app Prints exactly as designed
Best for Novels and non-fiction on e-readers, phones and tablets Forms, invoices, papers, manuals and anything printed

For a book people will read, EPUB is usually the better choice. For a document whose page design matters, or that will be printed, choose PDF.

How to open EPUB files

Here's how to open EPUB files on each platform. Unlike PDFs, they don't open in a web browser by default, so you need an EPUB reader app.

How to convert EPUB to PDF

Sometimes you need a PDF instead: to print a chapter, annotate it in a PDF reader or submit a manuscript. Our free EPUB to PDF Converter does this in your browser, without uploading the book. It reads the package file and follows the spine, shows the title, author and chapter list, and lets you pick A4 or US Letter, a font size from 10 to 16 pt, a title page, a new page for each chapter and page numbers.

Its limits are worth knowing before you start. The converter extracts the text: headings become bold titles, while paragraphs and list items are reflowed to the page. Images, tables, custom CSS and embedded fonts aren't carried over. Its built-in font covers Latin-1 characters, so most Western European accents work, but Cyrillic, Greek, Arabic, Chinese or Japanese text turns into question marks. DRM-protected books can't be read.

If you need images, styling or non-Latin scripts in the PDF, the free Calibre app is a better fit: its desktop converter renders the book's own HTML and CSS. For a text-first, private conversion of a DRM-free book, convert the EPUB to PDF online in a few seconds.

How to create or edit an EPUB file

Use real heading styles in your source document: most converters turn Heading 1 into chapters and the table of contents.

FAQ

What does EPUB stand for?

EPUB stands for electronic publication. It is an open e-book standard maintained by the W3C, and the current version is EPUB 3.3.

Is EPUB better than PDF?

For reading on phones, tablets and e-readers, usually yes, because the text reflows and you choose the font size. For printing or exact page design, PDF is better.

Can I read EPUB files on a Kindle?

Yes, if the EPUB is DRM-free. Send it with Amazon's Send to Kindle service, which lists EPUB among its supported file types (up to 200 MB per file) and adds the book to your Kindle library.

Why won't my EPUB file open?

It may be DRM-protected (for example, you downloaded an .acsm license file, which needs Adobe Digital Editions), the download may be incomplete, or your app may not support EPUB. Try Calibre or Thorium Reader.

Is an EPUB file just a ZIP file?

Yes. It is a ZIP archive with rules: the mimetype file comes first and uncompressed, and META-INF/container.xml points to the package file. Rename it to .zip to look inside.

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