Using PDFNet.dll (3.5) with WCF Library

Based on the error it seems that you are missing standard VC Redistributable on the targer server.
For more info, please see

If this does not helps:

a) If you are suing .NET 4, you should use PDFNet for .NET 4 and not version for 2-3.5
b) If you are using Any CPU project setting you need to make sure that you are using 64-bit version of PDFNet (i.e. it should match address space 32/ vs 64 bit of the containing process -