Linux - "open" does not open preview

[sguav]

On Ubuntu 16.04, open is actually openvt which in turn doesn't work for this.

Using xdg-open should do instead

...
 elif platform.system() == 'Linux':
      call(['xdg-open', pdf_filename], env=ENVIRON)            
...  

Not sure if this is just me, quite surprised no linux user found this before