Helps to stick to convention, that way there's little-to-no confusion. Kinda like using non-standard abbreviations, such as "pd" for "pounds", when you should've used "lb" (which comes from the latin word for weight, libra). Only place I've seen "pd" abbreviation is on older British field guns, where it's "pdr" (as in QF 13 Pdr, [Quick-firing 13 pounder] referring to weight of the shot).

Otherwise, "pd" means "per diem." Standardization and consistency is more important than folks realize. Blame it on "public education" I suppose :P