Altering Imperial measurements to Metric (standard solution not working)

Hello

I am getting overly annoyed, I work in metric units and for some unknown reason the pdf I open is in imperial, and for the life of me, I cannot change it.

The standard solution:

Edit->Preferences->Units->Page units [Millimeters] 

Is not working, it still measures my pdf in inches, the whole thing is super annoying and I am spending far too much time banging my head against the wall. 

So! What can I do?

(Note: the pdf was exported from Archicad where units are defined in metrics).