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PDF is an envelope that can have different things inside, sometimes only one kind of thing, sometimes layers per page can be different kinds of thing or different pages can be having different internals.
Images can be different kinds of vector, or generated at display time by postscript or raster images. Different kinds of raster images can be encapsulated.
PDF isn't much like an MS Word document, though more like that than an epub.
So there is no single approach to PDFs, it depends what is in them and the quality you want. They need different tools to images inside an epub.
Each PDF page has the size of page and what is in it, which layer it's on and where it is relative to the page.
See k2pdfopt, The GIMP, Imagemagick etc.
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