EpubCCWhat is a reflowable EPUB?
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A reflowable EPUB stores text and its structure, not positions on a page, so the reading app lays it out again for each screen, font and size. It is the default kind of EPUB and the opposite of a fixed-layout one.
Why it matters when you convert a PDF to EPUB
A PDF is the opposite: every character has a fixed position on a printed page, so a phone or e-reader can only shrink or zoom it. Converting to a reflowable EPUB means recovering what the PDF never stored explicitly: which lines form a paragraph, which text is a heading, what the reading order is and which lines are page furniture (running headers, page numbers) that must disappear.
Reflowable text is what makes font size, margins, night mode and dictionary lookups work on an e-reader. The cost is that the exact page design is lost, which is why some documents should stay PDF.
How EpubCC handles it: every EPUB it writes is reflowable. Each detected chapter becomes one XHTML file, paragraphs are rebuilt from the positions of the lines, running headers and page numbers are removed, and a short stylesheet sets only margins, indents and heading sizes, so the reading app stays in charge of the font, the size and the line length.
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See how EpubCC rebuilds paragraphs
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