As many people, I have many PDF (Portable Document Format) documents, especially technical ones.
I like reading, highlight text and annotate printed documents, but many times I simply don't want to waste hundreds of paper sheets! What a waste of paper, especially for new manuals for each software release...
So I decided to buy the popular handy software: FoxIt Reader.
The reader-only version is fast and also free, but I decided to buy the Reader Pro Pack version ($39) that let me insert/modify annotation, highlight text and do many other things.
Maybe there are some free PDF tools out there, but the software was easy to use, fast and not too expensive.