Sorry, I intentionally left out gory details about what was wrong about the original PDF document. I assumed that you probably do not want to know about technical details as long as the problem is fixed.
Because I do not know the "history" of the original PDF document, I am not able to determine how the original PDF document was partially corrupted/damaged. It is highly likely that the original PDF document already had the problem when you got it.
In any case, I believe that the problem like this does not happen often. I would recommend making a backup copy of the new version of the PDF document (the one I gave you) now for safekeeping. If it happens again for some odd reason, you should be able to use the backup copy.