I've seen this issue before. Looking at the
Nuget documentation [
^] for iTextSharp shows several versions. If you have 2 or more projects using that package, you need to ensure they all have the same version. If they have different versions, you'll get this error.
In VS2015 Solution Explorer, right click each project that uses iTextSharp and select "Manage Nuget Packages". The window that comes up will show installed packages. Click on iTextSharp and the right pane will be "Version" with a drop down next to it. Ensure that all projects are targeting the same version of the Nuget package. Once they are all the same version, re-compile and test, the error will go away.
Hogan