[Parody]

OK then. It looks like some or all of the document was created by Acrobat's built-in OCR capabilities and they didn't bother to check and fix the resulting text. Depending on the options they chose it may actually look like a mix of text and bitmaps, or it might appear as just the scanned images and the text completely hidden (only used for searching and copy/paste).

FWIW: I have not seen this in legitimately published book PDFs but I can believe that some publishers might not care to fix their work, especially for old books for which they no longer have either the source document (and all of its parts) or the application in which it was created and a system that can run that application (or that predate "modern" desktop publishing). :(

(Sorry if I'm flailing a bit; I don't OCR things but I do a lot of other desktop publishing work.)