Thanks for your inquiry and sorry for late reply as weekend. Sorry that trapping keys event (such as KeyDown) is not available for our PdfDocumentViewer at present. If they can be achieved in the future, we will let you know. Any other questions, welcome to write back.
Sorry there is no available workaround for you at this moment. Considering your situation, I have given your issue a high priority. Our Dev team will work on it. If there is any update, we will inform you immediately.