I use AutoCAD a lot and it may be worth noting that most of the time the scale is manually entered so there is always an element of human error. Furthermore, there are plenty of other things that can go wrong such as an incorrect viewport scale and how the layout is scaled in relation to the paper when converting to PDF.
Presuming that everything is perfect and there are no error in the creation of the PDF you may need to check if an option is either on or off in Adobe Reader or Adobe Acrobat found via:
Edit > Preferences > Measuring (2D) > Use Scale and Units from Docuemnt (when present)
If changing your current setting still doesn't solve your problem then you can setup your own scale ratio:
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 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