Assuming the PDF has already been converted to JPG
1.) Click Tools > Picture
a.) Check the box for Maintain Aspect Ratio - this ensures that the picture isn't distorted when it comes in
b.) Click Load From File, choose the JPG and click OK
2.) Click to locate the bottom left corner of your picture, click again to locate the top right - size is not important here, but ballpark as best as you can
3.) Select the picture, lower it in the Z plane (type ",,-1" to drop 1')
4.) Zoom into a measurable element in the picture - if there is a marked measurement, perfect, if not, you can use a doorway, assuming it's 36" The element that you are measuring should fill the screen so that you can get the most accurate measurement
5.) Measure the element - using the line tool. Click to start measuring, move the mouse pointer to the end of the measurement. Note the "Length" displayed in the bottom right corner of the screen then right-click to end the command without drawing a line.
6.) Select the Picture, then click Actions > Scale
a.) Check the Calculate Scale box
b.) Enter the Desired Distance (36" for the doorway)
c.) Enter the Actual Distance (the length of the line that you measured)
d.) Click Scale
This will get the picture very close to the correct scale. At this point, run through the above process again just to fine tune the JPG's size for accuracy.
When you're happy with the scale, put the picture on its own layer and lock that layer so that you don't accidentally select/stretch/move/delete it.