muddy point: access 2

Question: even if errors are found (Google books, badly scanned PDF) how much work is involved in fixing those errors? Once a PDF is created, is it possible to update one page within the file, or must the entire work be rescanned? If individual pages are individually linked as separate images this wouldn't be a problem, but for the PDFs I download, they are self-contained documents. Wouldn't it be less time/effort to more carefully review items before finalizing them, than to have to go back and re-digitize?