How to combine digital certificates with PDF files?

z3r0aCc3Ss · Post by **z3r0aCc3Ss** » 02 Jun 2010, 11:16

My question may be incorrect in English language, but what I meant to say is, I want to add some security to my PDF files.
I remember, few days ago, I got some PDF file and it had certificate along with it. I had to install that certificate in order to view that PDF. So that, only one copy will get installed on that computer and I can't copy it too. Even if I copy to some other computer, I get some error that certificate is not valid. That's a good security.
How to do that with our normal files? Is there any way or software to do that?