There is a very, very good chance that you can't. The .NET framework has a built in limitation, that no single object can exceed 2GB. Since iTextSharp is written in C#, it is quite possible that iTextSharp is hitting that limit, particularly if you are not having a problem with similar files that do not exceed 2Gb.
It may be worth having a look at the
iText website[
^] and seeing if anyone else is hitting this problem.