What is the EPUB File format?

1What is an EPUB file?

EPUB, short for electronic publication, is the most widely used open format for ebooks. Its defining trait is reflowable layout: the text is not fixed to a page of a set size, but rearranges itself according to the screen width, font size, and margins the reader chooses. That is why an EPUB is comfortable both on a small phone and on a tablet.

Besides text, an EPUB can include a cover, a navigable table of contents, images, notes, book metadata, and even audio and video in the most recent versions. It is usually distributed through digital bookstores, public libraries with electronic lending, and self-publishing platforms.

2History and standard: from EPUB 2 to EPUB 3

The format was developed by the IDPF, the International Digital Publishing Forum. EPUB 2 arrived in 2007 and became the common format for e-readers. EPUB 3 appeared in 2011 and brought important improvements: support for HTML5 and CSS3, better handling of languages with complex scripts, audio, video, math, and fixed-layout design.

Over time, stewardship of the standard passed from the IDPF to the W3C, which publishes the recent versions, such as EPUB 3.3. This means the format evolves alongside web technologies. The fact that EPUB is based on HTML and CSS explains why its files are easy to produce and adapt to different kinds of screens.

3How does an EPUB work on the inside?

If you change an EPUB's extension to ZIP and open it, you will see that it contains web pages in XHTML format (one per chapter or section), one or more CSS stylesheets, the images, and several control files: one that lists all the components and the reading order, another with the table of contents, and a third that indicates where the main package is.

The device's reader loads those pages, applies the stylesheet, and fits them to the screen, calculating where each page break falls. That is why the page numbers in an EPUB are not fixed: they change if you increase the font size. Some books also include a page list from the print edition, so you can cite the same page as other readers.

4EPUB vs. PDF: when should you use each?

PDF fixes the layout: every page looks the same on any screen or printer, which is ideal for contracts, invoices, forms, catalogs, scientific papers with figures, and any material where layout matters. The drawback is that on a small screen you have to zoom in and scroll around.

EPUB trades that visual fidelity for readability: the text adapts to the device and to reading preferences, and it is usually lighter. It is ideal for novels, essays, and manuals meant for continuous reading. For books with complex layouts, such as cookbooks or heavily illustrated children's books, there is fixed-layout EPUB, or people simply choose PDF.

• Long reading on a phone or e-reader: EPUB.
• Document that must be printed or look identical: PDF.
• Content with wide tables or plans: PDF.
• Accessibility with adjustable size and read-aloud: a well-built EPUB.

5How to read an EPUB on each device

On iPhone, iPad, and Mac, the Books app opens EPUB natively. On Android there are many free reading apps, and most e-readers from other brands recognize it directly. On Windows computers, you can use an ebook reader program or a browser extension. Many digital public libraries offer EPUB loans with DRM protection, which requires a compatible app.

On Kindle, Amazon's native format is different, but you can now send EPUB files through the Send to Kindle service, which takes care of adapting them. DRM-protected books from a store can only be read in the apps authorized by that store, and bypassing that protection is neither possible nor legitimate.

6EPUB, PDF, and Rayo PDF GO

Rayo PDF GO does not create EPUB files, but it helps you in the opposite direction, when you have content you want in PDF. If you want to save a web page or a long article as a fixed document, the Web to PDF tool captures it; and if you have a PDF of a book with very heavy pages, Compress PDF reduces the size so you can carry it with you. If the PDF is a scan, OCR adds real text and lets you search inside it.

When a PDF is too awkward on a phone because of its column layout, extracting the text with Extract text lets you turn it into an easier-to-read text file, at the cost of losing images and layout. All of this happens on your device, without sending your books or documents to third parties.

As for preservation, keep your EPUB files in an organized folder with the title and author in the file name, and back them up as you would any important document. DRM books bought from a store depend on the store account; if you ever change devices, you will need to authorize your account again to read them.