I'm trying to open a PDF file on the web and I get this message: Your PDF Viewer may not be able to display this type of document. What do I need to do? I have the PDF Reader Pro Free.

I'm trying to open a PDF file from the web  on my iMac desktop and I get this message:  Your PDF Viewer may not be able to display this type of document.  I have the PDF Reader Pro Free.  What do I need to do?