[Shamun]

Sorry to say, but trying to protect something you give to a customer is not very cost effective.
If it can be viewed on a computer, I could use the 'Print Screen' button and copy it into photoshop to bypass anything embedded in the PDF. If its a video type, then a screen recorder can be used.

The only decent way to protect your products is to do as Jean says. When you remove the anonymous part of the internet, peoples backbones melt and will either try to blank it out (easily done in photoshop) or just not distribute it.
I would suggest putting their name, email, phone and last four digits of the CC as a stamp across the entire document.

There's also another way of protection, but only really effective if you're willing to take people to court over it. Have some small typos or characters on the pages that are unique to that persons purchase. If your product is pirated, you can match up the characters to see whos copy it is, while the buyer is none the wiser.