Unfortunately, more times than not, I am forced to scale the converted to CAD file using a hospital door and guessing the width of the door, by standard door widths for a certain type of door. I usually get it 'close' for our needs, such as figuring how much Network wire we need down hallways. SOMETIMES, the scanned PDF will actually have a 'scale' in the corner that I can use. - I realize that this is Horrible, but again, I have to use what I can get AND the customer is willing to pay for.
Another option that I always try first.... IF there is a titleblock on the scanned dwg, I contact the firm that drew it and TRY to get an actual CAD version. Most of the time, they are willing to send me one. But other times, they want to charge me hundreds of dollars for each sheet, which of course I won't do. IF I can get to the actual CAD person instead of the Engineer, I typically have greater success.
Believe me, I have OCD also, (to a fault), but time IS money and it is VERY tight in my discipline. As a professional, I must be realistic.
craigr