I have a client who would like to sell/license digital content in the form of downloadable PDFs. After payment has been received, we would like to apply a unique watermark to each page of the PDF preferably with information that identifies the customer and the date of the transaction in a way that helps to mitigate unauthorized copying and redistribution. I suspect we'll be writing a custom module to do this but thought I'd start here to see how much we'll be reinventing the proverbial wheel. Thanks in advance for any tips and hints.