For those who want to copy protect PDF, the options are numerous and their effect on preventing copy can be less than ideal. While there is a lot of information on the Internet about copy protecting PDF, much of it can be disinformation due to the lack of research on one hand, and the misuse of buzz words to sensationalize on the other.

"DRM" stands for Digital Rights Management and the term was created to describe the control of access rights to use movies. Movie making is a billion-dollar industry and the piracy of movies is too common a practice despite it being illegal. A lot of different methods of preventing redistribution have been tried, including locking the video file to the CD or DVD disk by embedding the encryption key into a hidden sector of the disk. Special disks and disk copy equipment is required and the process can be as expensive.

However, what passes for DRM according to the makers of document protection software may not be complex at all. For example "password" protection is often considered to be "DRM" even though it does not prevent duplication of the file itself. Nor does it prevent sharing with friends because the password can be shared with the disk.

Distributing media on special disks may not be practical and delivery by email and download is a lot easier. So what is needed then is a DRM solution to protect such portable files that are economical? If the media file can be locked to a user's computer then it can only be opened on that user, making any copies useless to anyone else.

The media file needs to be encrypted and one of the keys for decryption needs to be a unique identifier, one that is peculiar to the user's computer. Some DRM solutions use the serial number of the computer's hard drive while others use the computer's MAC address. Unfortunately, MAC address differs between the network device being used such as LAN or WiFi, so it is not a stable constant and can escalate calls for support.

But either way, such DRM can be effective providing a proprietary viewer is the only one that can be used to decrypt and display the media while checking the user's unique identifier.
Such DRM can copy protect PDF and almost any other type of media. Why the movie industry doesn't use the same DRM is because it would require custom hardware, i.e: a video player with software that can decrypt the video after identifying the serial number of the video player itself. 
However, if you want to protect video for use on computers, this method will be ideal.