Preview Not Opening File - Permissions Issue (via Python)
I have tested this a bit in my free time today, and it seems that subprocess.call(['open','-a','Preview',path]) is your best option for opening PDFs in Preview from Python. 

I'm still not sure why using the absolute path to Preview doesn't work, though.

I am trying to find a way to get a process PID based on its window title in AppleScript. For example, if you were to open a document named "Test.png" in Preview with that command, it would spawn a Preview window with the window title "Test". I am hoping there is a way to get the PID of this process using the window title, but I'm not sure yet.