EpubCCWhat is a tagged PDF?
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A tagged PDF contains a structure tree that marks headings, paragraphs, lists, tables and the reading order, on top of the drawn text. Tags are what screen readers use, and PDF/UA requires them.
Why it matters when you convert a PDF to EPUB
When tags are present and correct, they tell a converter the structure directly. Many PDFs have no tags, or tags generated badly, so a converter cannot rely on them and has to infer paragraphs, headings and reading order from the positions and sizes of the text.
How EpubCC handles it: the engine does not read the PDF's tag tree today. It infers the structure from what every PDF has, the position, size and weight of each glyph, which also works for the many PDFs that have no tags or bad ones. A correctly tagged PDF therefore gets no extra benefit yet.
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