Hello, I sell pdf documents - purchaser has the right to copy once. Is there any way to ensure more copies are not illegally made and sold by purchaser?
More aboutcopyright issue photocopying problems
The only idea I could think of to possibly help you is to use copyscape. It's for web content, although it can also be used if someone has duplicated your content then published it online which would be the most likely scenario.
Some soft will do it, but in the end it's circumvented easily. As long as you cover your ass (1 copy per YOUR software, or Legal Agreement) you are safe. What they choose to do with their copy is none of your business