I know this sounds dumb and my boss hates it, but it works. xref a .pdf into your drawing. open a .pdf reader to check the scale. on the "attach pdf underlay" window locate boxs that says scale. if the drawing is a 1/4 inch scale uncheck the boxthat says specify on screen and type in 96. if the the scale is 1/8 inch type in 48. (the math is sound but if you disagree then feel free to check me) then move .pdf by means of a known point... 0,0,0 or a grid line that in on both your pdf and auto cad drawing. for a scaned .pdf the scale my be very off. so be careful.
Additionally you can scale your .pdf by reference. It works just like rotate by reference. Find four crossing intersections, two from the xrefed .pdf and two from known grid lines. I use circles.
1.Place a circle of different color on each of the points (ex. Column A-1 grid red, A-1 .pdf yellow, Column J-10 grid green, Column J-10 .pdf cyan).
2. Place the .pdf A1 on A1 grid lines.
3. Type _SCALE enter then select the .pdf. Select center of circle on A1 as base point.
4. On command line select REFERENCE. Select same A1 base point again.
5. Then select cyan J-10 (from example) on .pdf
6. Then select green J-10 on grid lines.
If your .pdf is square with the world then it will now be fairly accurately place in your .dwg.
If your .pdf is not square with the world start over and use this same method with rotate by reference to make it plum level and square.