transmission lines voltage drop
The formula

$$ \Delta V \approx P R + Q X \textrm{ [per unit]}$$

appears to be based on an assumption that the voltage drop will be relatively small.

I have excerpted a few slides from a presentation by Kashem Muttaqi, originally found at http://egpreston.com/VoltageRegulation.pdf , below. The slides explain how we get to the approximate formula.

(This was found on the third page of a search for voltage drop pr qx - apparently pr qx is a fairly unique "phrase".)