We managed to solve the issue, essentially in three steps:
1. Recompile the pdfSharp assemblies from source AFTER deleting all sources containing System.Forms references (very simple: they're all in the source folder PdfSharp.Forms)
Now you can try it in the Unity editor, which automatically loads all necessary & compatible dlls. Outside the editor you need to manually add them to the project:
2. Find a compatible System.Drawing.dll, see
http://answers.unity3d.com/questions/53170/using-drawing-package-like-systemdrawing.html for details, it should be somewhere here in the editor: %Unity Folder%\Editor\Data\Mono\lib\mono\2.0 . For Mac support you'll need some extra efforts.
3. Add the internationalization dlls from the editor libs (see 2.) as described here (I18N.dll e.g.) :
http://answers.unity3d.com/questions/42955/codepage-1252-not-supported-works-in-editor-but-no.html The only thing which doesn't work is web player support, but there may be an pdfsharp update in near future for that.