is one of the free open source tool available. Though if you are going to use your application for commercial purpose, then you have to either buy license or release your applicatin code as open source.
The source code and DLLs are at http://sourceforge.net/projects/itextsharp/
This is one of the best tool available. These are the two links available to create and edit PDF in C#.
Of-course, you will have to figure out your requirement yourself in terms use-case and all.
Hope that helps, If it does mark it as answe/upvote.