iTextSharp is a free and open-source library used for creating and manipulating PDF files in .NET and C# applications. The library is built on top of the iText library, which is written in Java, but iTextSharp is specifically designed for .NET applications. The itextsharp.dll file is the main binary file for the library and contains the code necessary to interact with the iTextSharp API.
If the itextsharp.dll file is missing or encounters an error, then the iTextSharp library will not function properly. In this case, the solution would be to either reinstall the library or repair the installation if possible. It's also possible that the error is due to an issue with the specific application that is using iTextSharp, in which case the application may need to be updated or reinstalled.