The two most popular file locking softwares are drumlinsecurity and locklizard. These two softwares can make the pdf exclusive to certain computer or IP address. This way, sharing of the PDF via torrent or the piracy is avoided. In order to get better grip on the piracy, some of the ebook hosting sites are now printing the name of the person and email to identity the ebook file. This way not many legit people share the pdf and piracy is restricted to some extent. I suggest checking out LockLizard if you want PDF security.