I have an old project that is using iTextSharp library for PDF generation. iTextSharp DLL is added as reference to the project. iTextSharp was originally available under the
iTextSharp (LGPL / MPL) 4.1.6 is available via NuGet.
For more information, see http://nuget.org/packages/iTextSharp-LGPL.
Right click on the dll and go the Properties, under the Details tab it the version number should be listed.
iTextSharp 4.1.6
has sources on github.
You can also check the PDF it generates' Document Info values, the "Producer" in particular. In Adobe Reader you hit ctrl+D and go to the first tab.
seems like iTextSharp maintainers removed iTextSharp 4.1.6 from SourceForge, anyway you can try to simply Google for "itextsharp-4.1.6.zip" to find alternative download sources
EDIT: here is the link for 4.1.6 dll: http://pdfviewernet.googlecode.com/svn-history/r55/trunk/PDFView/PDFView/lib/itextsharp.dll