Page page = doc.GetPage(3);
if(page.GetNumAnnots() > 0)
{
Annot annot = page.GetAnnot(page.GetNumAnnots() - 1);
}
This behaviour is part of the PDFViewWPF tools, which is another sample. You can copy this sample into your own project and use it as is. Once you create a PDFViewWPF, you have to create a ToolManager from the Tools sample and pass in the PDFViewWPF. You then need some buttons like the above ones to switch between tool modes. You can simply use ToolManager.CreateTool(ToolType.e_annot_edit) and it should now be possible to select annotations.
You can also modify the tools code however you like to suit your own program.
If you need additional help or clarification, don't hesitate to ask.
Best Regards,
Tomas Hofmann