Scale correct but measurement wrong

I've set the scale on the measurement tool but when i measure a known size on the drawing Adobe is showing a smaller size. Is there a way of calibrating the scale by measuring the known size on the drawing. I've previously used bluebeam revu, where you can do this to set the scale.

I think the issue has arisin when the drawing was converted from a cad drawing to pdf and therefore the scale shown on the drawing is not quite reflective of the size measuring from the pdf.