how to put permission to only paid users can print something in my drupal site I have a Drupal site which I plan on putting a print pdf function to its contents. How do I manage this so that only paid users can print the pdf?
If you mean controlling who can view/download (i.e. be 'served') the PDF then you need to read up on the private file system (Working with files contains a pretty good introduction).

If you mean showing the PDF to anyone who requests it, but only allowing paid users to print from it, you can't. That would be controlling a user's browser, which you can't do for obvious security reasons. It's the nature of the web; if you want to distribute content, you have to face the fact that it can be copied unless you go down the DRM route.

The only DRM solution I know of for PDFs is Adobe Content Server; it's written in Java and is a real pain in the proverbial but it does work. You can control the number of times a person can print or even copy/paste data from the PDF.

It does introduce a really annoying workflow, though, that you'll need to explain in detail to your users; and it ain't cheap.