To access Measurement tools within the radial menu, tap on 'Insert' then 'Measure'
To begin, calibrate a known length by tapping on the 'Calibrate' tool then measure out a distance and insert the known scale/measurement.
The units of measurement can be changed within the app Settings:
Once the PDF has been calibrated, you can measure out linear distances or rectangular/polygon areas.
The properties of these shapes can be changed in the layer deeper in the radial menu.