"p.p" = per pro. = "per procurationem". This means 'by/through the agency of".
If I'm your PA, and you're out of the office, I sign the letter for you and put p.p. before my name, i.e.
Beware of explanations that say that it means 'for and on behalf of', and should be put in front of the absentee's name...
PS: Welcome to UE, by the way!