A book gets reprinted if there is sufficient demand for it. I cannot define it any better or give any data (that's internal).
When a book is reprinted, all current errata is folded in, and if the PDF had not been updated previously, it is at that point.
PDFs that are not (re)printed are updated as time allows.
You can tell if a book has been reprinted by looking near the legal text, it will say 'second printing' or the like.