# What is EPUB media overlays?

Source: https://epubcc.com/glossary/media-overlays/ (updated 2026-11-04)

Media overlays synchronise recorded narration with the text of an EPUB 3, using SMIL files that pair audio clips with elements of the content documents. Reading apps highlight each phrase as it is read aloud.

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

A PDF carries no narration, so converting one never produces media overlays; they are added later, for read-along books. They explain the MED messages in EPUBCheck, which check that clips and text references match.

How EpubCC handles it: it never writes media overlays or SMIL files, because a PDF has no narration to synchronise. An EPUB from EpubCC can still be read aloud by a reading app's text-to-speech, which works better than with the PDF because the text is in reading order, the language is declared and running headers are no longer read out on every page.