Some PDFs consist only of pictures of scanned pages... So 200 page book has 200 images in it. Others, mostly newer ones, have text and pictures inside the PDF.
The US Army has several books in the Green Book series that are just like what itimpi is describing, blocks of text and images of blocks of text. There is no reason why some is text and some images of text, except perhaps that someone put Acrobat on some automatic setting and that is what it came up it.
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