Does anybody know a good way to protect a data cd full of .pdf files?

[tonyG]

lets be honest...
if what you were asking was possible then sites like this would not exist!!!

however if you have some programing abillity and you are desperate to stop people with no knowlege about copy protection try the following...

to prevent direct cd copy make intentional errors
ie have a large file at the end of the cd that you are copying and turn off the computer before the burn is finished is will give no brainers a hard time

to prevent pdf files from being accessed while not on the cd, embed the files in a viewing program that ##### certain aspects of the source and will return an error if these aspects are incorrect.

however if i was you, i wouldnt bother its to much effort for too little result...