Experience Writing Code to Read & Modify PDF Files?

Do any of you folks have any experience writing code to read and modify PDF files? Obviously, one would use a suitable preprogrammed package for this, and there are some such, but that's sometimes harder than it looks. If any of you can offer some tips (for a worthy cause), for instance which packages are best and how they can best be used, that would be great; please e-mail me at volokh at law.ucla.edu. Many thanks!