How to make file downloads accessible by only some users in your system

What is a good way for making only certain users in my web application able to download a certain file? For instance, if a user purchased a pdf on my website, what methods could I use so that only the people who purchased this pdf could download it? I've read that I could store file names as GUID's and then just rename them upon download. I am looking for a method that is secure, prac开发者_开发问答tical and would work with amazon s3 web services.