Every PDF takeoff session starts with calibration. The drawing's title block usually states the scale (1/8" = 1'-0", 1/4" = 1'-0", 1"=20', etc.), but the printed/displayed scale doesn't always match — PDFs can be resized, scaled, or rotated. Calibrating from a known dimension on the drawing guarantees accurate measurements regardless of how the file was prepared.
Each drawing sheet has its own scale; some sheets within the same set use different scales. Calibrate each sheet separately. The site plan might be 1"=20', the floor plan 1/8"=1', the wall section 1"=1'. Mixing them up generates wrong quantities.
BidScreen XL's Scale tool handles this — snap to two points, enter the actual dimension, and all subsequent measurements on that sheet use the correct scale. Switch sheets and the calibration persists per sheet.