I'm trying to make a feature in my program to let the user measure
lines on a pdf. Say there is a line 4 feet long on a pdf file which
would of course be scaled down to fit in the window area. If the user
clicks on one end of the line and then clicks on the other end, the
program would then tell the user that the distance covered is 4 feet.
By clicking only halfway down the line, instead the program would say
it is 2 feet long. How do I go about doing this?

It would need to be able to find the scale of the pdf file so that it
could correctly estimate the distances.

Thanks!