I ported the current version of PDFSharp to Xamarin.iOS to reuse some existing code for extracting text from PDF documents on an iPhone.
To build simply open the solution in Visual Studio (if you have Xamarin.iOS Professional) or Xamarin Studio und build the PDFSharp_ios project.
As I needed this library mainly to extract text from pdf files. No warranty is given that it works equally well to create them. You may need to improve the XFont.cs and XFontFamily.cs files to make it work. Working extraction has been verified by using Philip Trelford's F# snippet here and comparing the output on Windows and iOS.