Software for copy protection

A friend wants to copy protect his teaching material.

This need not be foolproof as his customers are not necessarily black-belt hackers.

I have the idea to include a not easily removable fingerprint into the files that identifies the customer: in case of an exemplar pops up on the Internet, my friend could identify who put it up.

What open source software would you recommend for this? Preferrably on Windows. The format is Microsoft Word .doc .