Regardless of the printed scale of each page on the plan (or even if it is not to a particular scale), you can scale the plan using a known measurement on the plan. It is best to use a long dimension for better accuracy.
First Click the Scale button 
from
the toolbar
The Scale Dimension dialog appears.
Next type the measurement of a dimension shown on the plan.
Click OK
Then click the two end points of the dimension line.
The
Scaled Icon 
now appears
in the Pages Window
You can scale both vertically and horizontally for improved accuracy, in case the original printed document was out of proportion or stretched. To scale both ways, you just follow the scale procedure again except choosing a new dimension going the opposite direction as the first dimension that was scaled.
View demo video on how to scale a plan.