Zip is zip; there is no big difference what to zip, PDF or not (compression rates and optimal parameters may vary though).
Please look at these two libraries for .NET:
Open-Source DotNetZip library,
http://dotnetzip.codeplex.com/[
^].
A .NET Wrapper for the famous 7-zip is offered in this CodeProject article, "Best C# article of June 2008" prize winner:
C# (.NET) Interface for 7-Zip Archive DLLs[
^].
—SA