So what is to stop them from modifying the data to say whatever the people doing the modification want to to say?
The page views of the PDF version are photographic. Modifying it in such a way that the changes wouldn't show is not trivial. It would be an immense amount of work. Always download PDF versions of books that consist of photo page views of an original paper edition. Never download books that are in EPUB or MOBI format. Those can be easily changed.
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