2D Takeoff allows you to specify a sheet scale for files that may not already contain a scale, such as DWFs exported from PDF or image files. To specify the scale, you measure lines in a drawing, and then set the scale based on an original model scale, or a custom scale that you can specify. This can help calibrate your 2D takeoff to resolve scale distortions, if they are present in your files.
When setting a scale, you should measure lines that you know the length of (from an associated model or sheet). You then manually enter the known length of the lines, and the average scale is calculated for your 2D sheet from these measurements.