Another option would be to have your PDF stamped with the customer detilas as a watermark (on the fly while they are buying), Since the name of the customer, email (whatever you decide)and so on would be engraved on the documents, it is not as tempting to distribute, and if so, you'll know exactly who is doing it.
Check out
http://www.setasign.de/products/pdf-php-solutions/setapdf-stamper/
Just another suggestion...