XFINIUM.PDF is a .NET/Mono library for cross-platform PDF development with a wide range of capabilities, for both beginners and advanced developers.

Developed entirely in C#, this PDF toolkit that has been 3 years in the making, having started out as a custom-built library for a client and subsequently re-written as a common API for all .NET and Mono platforms.

XFINIUM.PDF allows developers to:

• Create PDF files and PDF portfolios
• Create, fill and save PDF forms
• Merge and split PDF files
• Perform content encryption and redaction
• Convert a multiframe TIFF image to a PDF file
• Add watermarks to an existing PDF file

It also has vector drawing capabilities and provides a built-in  barcode engine.

This sample from the Silverlight version adds annotations:

(click to enlarge)

The three editions:

• WIN/MONO Edition - for .NET 2.0 to .NET 4.5, Windows Forms, ASP.NET Webforms and MVC, Mono

• WPF/SILVERLIGHT/WINRT Edition - for Windows Presentation Foundation 4.0 and 4.5, Silverlight 4.0 and 5.0, WinRT (Windows Store apps)
  
  
• MOBILE Edition - for Windows Phone 7 and 8, Mono.iOS and Mono.Android.

are all combined in a cross platform bundle.

Don't be fooled into thinking that PDF is on its way out because plugins to render PDF in a web page are being replaced. The need to replace them is more about plugin security than it is to do with anything wrong with the PDF format. The ability to work with PDF in a general app is not only useful it opens up new possibilities.