The following features and settings are available in both Acrobat and the new Adobe Reader XI which is free.
To change your default page units:
This will enable millimeters to be displayed when measuring the page itself or displaying the paper size in the print options etc. We will set the scale ratio for the measuring tool later.
Ctrl + K > Units & Guide > Units
Set 'Page & Ruler Units:' choose 'milimeters' from the dropdown
To enable 'Use Scale and Units from Document when present':
Ctrl + K > Measuring (2D) > 2D Measuring
Tick 'Use Scale and Units from Document when present':
To change the scale ratio and units:
View > Tools > Analyze
Left click on the 'Measuring Tool'
BEFORE using the tool itself i.e. before you start measuring:
Right click > Change Scale Ratio (top of the list)
Change the settings here but note that the Acrobat/Reader will not remember your changes to the Scale Units but your other preferences with be saved.
If your drawing is say 1: 1000 that would be the same a setting 1mm = 1000mm
But maybe you want to display metres which in that case you would change it to say 1mm = 1m. Note that you can choose an appropriate unit from the last dropdown.
Or if no scale is present but you know the real world dimensions between two points then you could measure between the known points on the drawing; making sure you are set at 1:1 first and then choosing an appropriate setting.
i.e. a wall being 10mm when measured in the PDF but is actually 10m in real worl units, then you would change it to say 10mm = 10m
Don't forget that these will change the scale units for the Area Tool and the Perimeter Tool (available in the top right when Measuring Tool is in use.