.pdf organizer

[Moondrop]

I have a plea for Google, or any computer wiz out there: I need a program that will allow me to sort my .pdf files. As a student I have hundreds of articles, and a simle Microsoft-folder-organizing just isn't good enough. If the article is written by multiple authors I will have to choose which one to sort the article by, and my "system" might not appear as logical when I later try to find it again.

What I need is a program that allow me to attach key-words to the article, and let me search it up by using those key words. That way I might search author names, topic, school of theory, title, etc. With a topic-search I might also get articles with similar contents. An automatic key-word attachment will be appreciated. I don't want a universal search engine, but a simple program, where I can sort my articles, and that doesn't scan the entire hard drive to get what I need.

If such a program already exist, please point me in the right direction, and be appointed my sovereign Hero

I guess I should specify that I'm looking for a freeware program. All I can find seem to cost a lot of money -- that I don't have