I downloaded the PMBOK Guide Fourth edition again (lost my previous copy) and found something very interesting.
PMI has embedded 2 new anti-piracy features in the PDF Version of PMBOK Guide 4th Edition to prevent illegal distribution of the standard.
First, the file is password protected. Each time you open the file, you need to enter your PMI.org password to view the contents. In the file that I downloaded couple months ago, you had to enter the password only once and the file was unlocked forever. Not anymore.
Second, the footer of each page contains the name and PMI ID of the member who owns the copy. See the screenshot below.
I think it's a good move and would help prevent illegal copying and distribution of the guide to some extent.
However, anyone who's serious about PMP Certification, would be a fool to look for an illegal copy anyways. Read this post to find out why.