Hi,
to create a customized iOS and Android Xamarin module, you should update the modules downloading the latest versions.
For iOS you can edit the RadaeeLib project (located in ios/iOS/RadaeeLib) and use the MakeFile (call command "make" from Terminal) to create the new library and replace the existing one (located in ios/RDPDFViewXamarin/RDPDFViewXamarin/libRadaeeLibSDK.a).
For Android you can edit the latest Android Studio demo project (now it's the version 3.50), then build it and copy the aar file (located in PDFSample/ViewLib/build/outputs/aar/ViewLib-release.aar) and replace the current one (located in Xamarin module: android/RadaeePDF-Xamarin/Jars/ViewLib.aar)