We have a number mobile clients who are successfully using PDFNet with Xamarin Mono on iOS and Android.
Because PDFNet supports both C and C++ binding besides platform specific API (e.g. Objective-C or Java binding) you can access PDFNet in a platform independent way
via C-Interop (http://docs.xamarin.com/ios/Guides/Advanced_Topics/Native_Interop) or by using the platform specific APIs (e.g. see http://docs.xamarin.com/ios/Guides/Advanced_Topics/Binding_Objective-C_Libraries).
When it comes to .NET or Windows 8, PDFTron offers true .NET / WinRT (http://www.pdftron.com/pdfnet/mobile/windows8_winrt_pdf_library.html) components so you can directly access all APIs/controls from C#.