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There can be any number of layers of bitmap image, vector, postscript, java, text, fonts etc. PDF is an envelope with instructions about the page size(s), index/TOC, and how to display it all on a screen so it exactly matches the paper printout.
The text layer on most image based PDFs is just poor unproofed OCR, maybe by Finereader/ABBY etc. It's only so text search will work, which is all Google cares about.
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