I know this off topic (broadly defined), but it may help you to know that some PDF's produced from CAD programs can sometimes be retrieved in vectors / layers. I've used Inkscape for this, in the past. The results usually need some clean-up, but its often a good start point.

create a square that is a 1/4 by 1/4 somewhere next to your content. Select all Use the tape measure tool. click on one corner of your 1/4 x 1/4 square click on the other corner type the characters 1 & ' and hit enter. You're good.

@mr. doug said: I'll be popping in from time to time asking seemingly stupid questions A very wise man once told me that the only stupid question is the one you didn't ask, and it wasn't the question that was being stupid. Welcome aboard.