[ from WORLD WIDE WORDS, ISSUE 543, Editor: Michael Quinion, http://www.worldwidewords.org/ ]
Q. I would like you to tell me about the origin of the abbreviation "pp" when someone signs on behalf of someone else. I've heard that comes from the Latin "per procurationem", but is it true? [Anthony Vamvakidis]
A. This may be an unfamiliar business abbreviation to Americans, as I believe it is not much used there. It's commonly placed alongside a signature to show that it is being signed by somebody other than the ostensible author, say by a secretary in the absence of the writer.
Back when the abbreviation first began to be used in business, in the latter part of the nineteenth century, it was indeed taken to be a shortened form of the Latin phrase "per procurationem". But reference books today often say that it's actually short for the Latin "per pro", and that's how I learned it when I first came across it.
This is a more important difference than one might think. "Per procurationem" means "through the agency of" or "by proxy", while "per pro" means "for and on behalf of". Which meaning you take changes where you put the abbreviation. If the former, it should be alongside the name of the person who actually signs the letter; if the latter, it should precede the name of the true writer. Most people these days would assume the latter.
Even in Britain, an alternative form such as "dictated by Y but signed in his (or her) absence" is now common.
For completeness, it's worth noting that there's a third Latin phrase with some similarities, "in pro per", in full "in propria persona", meaning "in their own person". This is required in some legal jurisdictions to show that a person is handling their own case, without a lawyer. And "pp" may be more familiar in the sense of "pages", a nineteenth-century abbreviation from the English word "page" in which the doubling of the letter represents the plural.