Conditional "Jump to URL" Expression

I am successfully using the following expression in the Jump to URL field. Clicking on the PO number hyperlink opens a PDF of the purchase order that is stored on a shared drive.

="file:\\Servername\folder\POs\" + Fields!Customer_PO_Number.Value + ".PDF"

Unfortunately, sometimes the PDF  has not been saved to the shared drive. 

How do I change this expression to only jump to the PDF if the PDF file exists on the shared drive?

SSRS2005

Many thanks.