Short Answer: Yes, you can use iTextSharp 4.1.6 (please read important information about this particular version 4.1.6 below).
iTextSharp is the mature and solid PDF generation library for .NET, originally ported from iText, Java library. This open source library delivers functionality to create, modify, stamp, fill PDF files.
Question: can I use this library in a closed source commercial applications for free of charge?
Answer: this is allowed with iTextSharp 4.1.6 version (or prior versions) only as they distributed under Mozilla Public License (aka MPL) or alternative GNU LIBRARY GENERAL PUBLIC LICENSE (aka LGPL)
These type of licenses allows to use iTextSharp 4.1.6 in closed source applications and the only limitation is that you should put the notice (see the source code for more information about required notice) in About box or in the documentation for your application. You should also publish any changes from the original code if you have made any.
Question: where can I download iTextSharp 4.1.6 ?
Answer: you can download it from (these files were downloaded from original SourceForge links for iTextSharp 4.1.6):
iTextSharp 4.1.6 DLL only: itextsharp-4.1.6-dll.zip
iTextSharp 4.1.6 Source Code (C#): itextsharp-4.1.6.zip
Original Links for iTextSharp 4.1.6:
iTextSharp 4.1.6 - Binary (dll only) - download from SourceForge
iTextSharp 4.1.6 - Source Code (C#) - download from SourceForge
Question: what about later versions of iTextSharp (4.1.7, 5.xx and newer)? Can I use newer versions in commercial closed source applications?
Answer: Version 4.1.7 and newer versions are licensed under AGPL license which requires you to publish full source code of your application.
However commercial licenses for for use commercial closed source are available for purchase from 1T3XT BVBA, please visit this link to purchase licenses for iTextSharp 4.1.7 or newer versions: http://itextpdf.com/terms-of-use/index.php
Question: where can I get a commercial technical support for iTextSharp?
Answer: .NET Bear believes you can get technical support for this excellent library by purchasing a commercial license, for more information please visit: http://itextpdf.com/terms-of-use/index.php
P.S.: Oh well, thank you .NET Bear but what about converting PDF to images or extraction of data from PDF files? - Yes, at ByteScout we have commercial PDF Renderer SDK and PDF Extractor SDK for these purposes.