iText is used by Java, .NET, Android and GAE developers to enhance their applications with PDF functionality. iTextSharp is the .NET port.

iText is a PDF library that allows developers to create, adapt, inspect and maintain documents in the Portable Document Format(PDF). By utilising iText, developers can generate documents and reports that are based on data from an XML file or a database. Also available is the ability to create maps and books, exploiting numerous interactive features that are available in PDF. Developers can also add bookmarks, page numbers, watermarks, and other features to existing PDF documents.

iTextSharp key features include:

• PDF generation.
• PDF manipulation (stamping watermarks, merging/splitting PDFs).
• PDF form filling.
• XML functionality.
• Digital signatures.

Overall, iTextSharp is a great application that houses some really useful tools. For instance, developers can split or concatenate pages from existing PDF files, fill out interactive forms, and serve dynamically generated or manipulated PDF documents to a web browser. Pretty much anything a developer could need from a PDF functional application, iTextSharp does.