To begin with your final exhortation: because it isn't already manipulable. Elaborating on Joboy's previous post: it seems to me that you may have missed the following: PDFs may contain both text and images. Depending on the PDF(author)'s settings, you may select and copy those elements. JoBoy is referring to image elements which depict text, but can only be selected and copied as image. It's those text images he'd like to convert to editable text.
OK, now I get it. Merci.
