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.