PDF not scaling correctly

Hello,
My problem is when I load a new PDF and scale it using By Ratio, I enter it according to the scale on the PDF ex: 1" = 20' it does not scale correctly on the print. I'll place my cursor on a point, press and hold M on my keyboard, move my cursor to a known distance and it'll show it as a measurement that is about 1/3 of the distance. When I scale using the By Length option, again using the scale on the PDF, it will scale correctly until I move the screen so I can see another section of the print or I zoom in or out, then it is no longer scale correctly.
Any ideas on how to fix this?  I have tried different PDF's and it's the same on all of them.