Adobe Reader Xi Measuring Tool. Is the distance measuring tool. Acrobat is the more robust of the too plus its cross-platform so well start there.
In Acrobat X and XI. Ctrl K Units Guide Units. Ctrl K Measuring 2D 2D Measuring.
Set Page Ruler Units choose milimeters from the dropdown.
By the way Acrobat has had measuring tools since Acrobat 8 pro but the location and function of the measuring feature may be different in versions prior to Acrobat X. Click the tool menu then click Analyze and click the Measuring tool. Open the Properties Bar select the lines placed by a 2D measuring tool. Click on Measure and jump down to step 4 Click on Measure and jump down to step 4 Click the Add button.