Usage of SharpZipLib with GPL with Classpath Exception license the way you did it and releasing PDFSharp under MIT license is kinda defendable, but not entirely doubt-free. It has raised a lot of doubts whether the whole work shouldn't be licensed under GPL
Fortunetly there is no need for this situation to continue. SharpZipLib has been relicensed under MIT over a year ago:
https://github.com/icsharpcode/SharpZipLib
Please reintegrate files from MIT-licensed SharpZipLib into PdfSharp and allow wider usage without doubts.
I think it should be priority. Overview of code differences between GPL version and MIT version indicate only minor changes, so overall it shouldn't be very time consuming. I was able to get it to compile in 5 minutes, but it requires more tests and review of someone, who knows it better.