There's really nothing you can do aside from rely on the purchaser's sense of right and wrong.
Even if you password protect the file, if the purchaser is willing to share their purchased file, they're likely willing to share the credentials too.
PDF was designed to be a portable, shareable file type.
The only real way to protect a PDF file from sharing is by using DRM technology. However, that means that the user has to be connected to the internet to be able to view the file, and it also costs a lot. Any other form of protection is quite easily defeated by someone willing enough to do so.
As Jon already mentioned, there is not much you can do unless you want to go the route of DRM (Digital Rights Management). Companies like FileOpen provide additional security to PDF files, and only the user who purchased the file will have the right to open the document. DRM solutions are not cheap. You can find out more about FileOpen here: https://www.fileopen.com/
If you want to stick with plain PDF, one option is to add the purchaser's information to the document ("Document purchased by John Doe on 11/11/2016 from IP address 192.168.1.1") - if you want to be really certain that nobody will share your document, just add the credit card number that was used to purchase the document - you may want to check with credit card companies about the legality of this approach first.