Access path of open file
You can create small Apple Script and save it as an app. 

on open {dropped_item}
    tell application "Finder" to set the clipboard to the POSIX path of the dropped_item
end open


Place this app on the Dock (easier to use). Then inside your pdf viewer you have small file icon on the top title bar -> drag it on top of the newly created app. With this, file path will be copied to the pasteboard.