I have Zorin installed on both my desktop, an HP Prodesk 600 and my Dell Latitude 3160 laptop. I have adobereader 9.5.5 installed and working on both.
Here is where it gets interesting. I was able to set Adobe reader as default to open PDF files on my Dell laptop by right clicking on a PDF file to get to properties, then open with, then set as default. When I try the same procedure on the HP desktop, Adobe reader does not even show up as an option to pick as default.