My company needed a library to enable us to programmatically modify PDF files. We found Aspose.PDF for .Net and it turned out to be exactly what we needed. It had every feature we needed including populating pre-defined form fields in a PDF file, creating form fields, adding bookmarks, encrypting / decrypting PDFs, and much more.

Aspose is also remarkably easy to use. The documentation was thorough and easy to follow, but the API was so simple and intuitive that we barely needed to consult the docs. All we had to do was reference the Aspose library in our project, and from then on, it was as easy as using a native part of the .Net framework. We figured out most of what we needed with nothing but Intellisense, but when we did need to consult the documentation, it was easy to find what we needed.