We have tried reproducing the described behavior with the latest version or RadPdfViewer but without any success. I am attaching a sample PDF file with text and an SWF video file showing how we have tested the selection on our side. You may easily open this video file by simply drag-and-drop it in some browser (Internet Explorer or Chrome for instance).
If you still reproduce the selection issue with the latest version of the product you may send us a simplified sample project with detailed steps to reproduce the misbehavior or a sample video showing these steps. This way we would be able to reproduce the issue on our side and find the cause of it.
I hope this helps. If you have any other questions or concerns please contact us again.
Telerik by Progress
Do you need help with upgrading your WPF project? Try the Telerik API Analyzer
and share your thoughts!