Capturing mouse coordinates in axAcroPDF control

Hi,
I'm using the snapshot capture tool to measure an area of a PDF document oppened in an axAcroPDF control in a windows application in VB.NET 2005.
However, I need to get the document coordinates of two corners of the area captured so that I can later draw a rectangle on the PDF file that shows the measured area.
Any ideas on the simplest way to do this?
Thank you