I found this thread on a search, as I have the same issue. Piggybacking on this question, this does work assigning a unique number, however for me, that PDF is e-mailed to another group as well as saved by the user. But now, when either of those people open that PDF, the number continues to incriment. Is there a way to make it not change once the PDF has been saved/e-mailed? My thought would be to make the entire form become read only, but I am not sure that would even fix it?