@Gatsby:
I think your PDF file password could be the problem. It's got some characters that work as special characters in a URL, and since you need to put the password as-is into your Payment Variable Info URL for the PDF stamping, that isn't going to work. Try changing your PDF password to something entirely alphanumeric, or at least limit your use of symbols to these: $-_,+!*'().
@MatthewCornell:
Not sure I follow what you mean? Caveat #4 only affects the buyer's original download link. If you send out an update newsletter with fresh links, or use the "Send free download link" feature in Seller Admin, that sends a new link for the most current edition of the file.