If you cannot preview you statements and/or snail mail communications, you probably have an active pop up blocker on your computer. This can even be the case if you can preview contact labels, but not the statements themselves. 

Previewing statements from Statement report or via the contact record:Previewing snail mail communications via the Communication Dashboard:

 How can I preview these files, not only the contact labels? 

Make sure you turn of your pop up blocker: Click the three little dots in the top right corner of Chrome > More tools > Extensions

Then toggle off all extensions active on that site that are related to Anti-Virus or Pop up blocking. Sometimes you will also see a little Pop up blocker icon next to your search bar (McAffee, AdBlock, etc.) where you can turn off pop up blockers for your ezyVet site.