Make sure you add the Itextsharp dll to the global assembly cache (GAC) on your server.
For 2010 the GAC is found under
C:\Windows\assembly
For 2013
C:\windows\microsoft.net\assembly\GAC_MSIL
(But to add an DLL for 2013, please follow the instructions on MSDN, you can not just drag and drop as in 2010)