Add Password to existing PDF

I have a client that sells access to a review service. Part of the content of that service is a series of PDF files. They want to protect, as much as possible, the pdfs.

The PDFs are already served via script, and are not directly accessible from the web. You can only get to them if you log into the site.

They want to prevent people downloading the files and giving them to other people. I’ve told them that there is no way to prevent this. They still feel that there is more that can be done to protect their content.

The only other option that I can come up with that we are currently not implementing is to have the pdfs password protected.

So, on to my question.

Can any give me, or point me to, a way to dynamically add a password to a pdf file that already exists? Basically, I want to take the pdfs that are currently on the server, and add a unique password to the file just prior to it being downloaded.

I know that, in reality, this is going to do very little to curb the problem and that the passwords can just be distributed with the pdf or can easily be removed. But, it might discourage a few people for giving the pdfs away.

Thanks for the help.

A quick google search for php password protect pdf reveals that there are many solutions to this.

Take the first result for example.