I create and sell interactive PDFs to my subscribers. I encrypt all my pdfs with passwords which only subscribers will receive once they've made payment. But recently I've discovered that some of my subscribers have been emailing the pdfs along with passwords to other non-subscribers, so they can have access to my material without payment.
Is there a way to bind a pdf to a single computer or user?
If not, are there any suggestions on how to counter this program?
You need a DRM solution for that, like Adobe LiveCycle. It is expensive, so you will have to decide whether your volume justifies it.