We are supplying one PDF file to various educational institutions, and we would like the PDF for each institution to have its own unique hidden code.
I was wondering if it was possible to insert hidden metadata into a PDF eg. a ten digit number, which might be visible to the user, but cannot be changed.
Sure. Just add your custom metadata value and then apply a security policy that prevents editing the file.
It's not 100% secure, but it should work for most people.