# What is the OPF package document?

Source: https://epublab.com/glossary/opf/ (updated 2026-11-03)

The OPF (Open Packaging Format) file, or package document, is the XML file that describes an EPUB: its metadata, the manifest of every file and the spine that sets the reading order. Reading apps find it through META-INF/container.xml.

## Why it matters when you convert a PDF to EPUB

When a PDF becomes an EPUB, everything the converter recovers ends up declared here: the title and language taken from the PDF, each chapter file in reading order, the images and fonts, the cover and the navigation document. A mistake in the OPF affects the whole book, which is why OPF messages are the largest group in EPUBCheck.