Thanks in advance for reading this message.
My problem is that I need to write a program that must list the pdf files in a folder that contain a text pattern.
The user writes a text and the program list the files that contain the pattern.
I know that there are a lot of free libraries that work with pdf files, but I have not tried none.
Can you recommend me some free library.
Thanks a lot.

You could do it with this: http://itextpdf.com

It's free, powerful, and highly flexible. You should read their book to know how it works, but there is plenty of resources on the internet.