PDF stamping is a great feature but it is only half implemented. It needs to work retroactively and also needs to work repeatedly.
For example, if I make an instruction PDF and sell it (being stamped) and later the information is updated with a new PDF, that PDF needs to be stamped as well for all customers (not just new customers) and that is not currently how it works. Currently if the pdf is updated, it is not stamped for all existing customers.