EPUB adapts to the reader
EPUB packages structured web content, metadata, navigation, styles, images, and fonts into an ebook container. In a normal reflowable book, the reader can change the typeface, font size, line spacing, contrast, and screen orientation. Chapters and paragraphs remain meaningful even though page numbers and line endings change.
This adaptability supports small phones and many accessibility needs, but it limits pixel-perfect art direction. Design the hierarchy and semantics, then let the reading system make the final page. Test more than one app because rendering engines do not implement every style identically.
PDF preserves a manufactured page
A print PDF fixes the page size, text positions, image placement, margins, headers, folios, and page breaks. The printer manufactures the book from that geometry. A PDF whose pages are the wrong trim or whose fonts are missing is not repaired by selecting the correct settings on the retailer upload form.
The same fixed behavior makes an ordinary PDF uncomfortable as a phone ebook: readers pinch, pan, and encounter type that cannot reflow. Reserve fixed-layout ebooks for content whose spatial relationships are essential, such as some picture books or technical layouts, and follow the retailer's special workflow.
Navigation and accessibility differ
A good EPUB has semantic headings, a navigation document, reading order, language metadata, text alternatives, and meaningful links. A print PDF needs logical pages and embedded fonts, while an accessible digital PDF requires additional tagging work that a print-ready file may not contain.
Do not claim that passing a validator makes a book accessible. EPUBCheck checks specification conformance. Human testing still needs to inspect reading order, link purpose, alternative text, zoom behavior, contrast, and whether decorative styling obscures meaning.
Generate both from one source
Maintain one structured manuscript and one set of semantic roles, then apply format-specific presentation. Print adds fixed page geometry, running heads, folios, and spread decisions. EPUB removes those physical assumptions and retains flexible text, navigation, notes, and media.
When the manuscript changes, regenerate and reproof both. The print change may alter page count and spine width, while the ebook change may affect navigation or notes without any concept of a spine.
Sources: W3C: EPUB 3.3 · W3C EPUBCheck · Amazon KDP: Supported eBook formats · Amazon KDP: Paperback submission guidelines. Reviewed ; recheck current platform requirements before production.