Choose a reliable reference
Find a clearly labelled dimension on the same drawing you want to measure. Use the actual ends of that dimension, rather than the edges of its printed number. A longer, clear reference usually makes small clicking errors less significant.
A printed scale such as 1:100 may no longer be accurate after a scan or resize. Calibrating against a known length ties your measurements to the drawing as it appears in the PDF.
Set the scale from the reference
Open Scale PDF, choose the page, and zoom into your reference. Use calibration to mark its two ends, enter the real length and unit in the dialog, then apply the scale.
For a dimension labelled 4200 mm, enter 4200 and choose mm, or enter 4.2 and choose m. Mixing the number from one unit with another unit produces an incorrect scale.
Measure a second labelled dimension to check the calibration. If the result disagrees noticeably, check your reference points and units before adding more measurements.
Measure and export
Use distance measurements between points. For an area, mark the boundary corners and close the shape by returning to its first point. Follow the intended outline; crossing the boundary lines creates a different shape.
Review the measurement labels on the downloaded PDF and keep the original plan. Check calibration separately for a different sheet or drawing detail: one page can contain views at different scales.
A distorted scan can have different proportions horizontally and vertically, so a single reference may not validate the whole drawing. Confirm important dimensions against the source plan; these results are not certified survey or construction measurements.