PDF identification / regenerator

A client of me has some pdf files. He wants to sell them to his clients. He wants when the client has paid for it, the pdf is regenerated and that a barcode or code with the name of the client is added to the pdf as watermark. In this way the owner of the paper should be unique. It should be possible to put the tool in all e-shop applications. who has the best solution for this? Be carefull, we aren't interested in general bids. If you can provide an example it would be great. So be concrete. Thanks Luk