Here is the Oxford English Dictionary:I suspect PAX is may well come originally from the airline industry. Abbreviations ending in X are quite widely used: WX for weather, CANX for cancelled and so on. I think these may in turn have originated in wireless telegraphy (in morse), where X abbreviations are still used (particularly by ham radio operators), for example TX for transmitter, RX for receiver, DX for distance.
(My wife used to work as flight operations secretary at Plymouth airport and still writes CANX on our kitchen calendar to indicate events that have been cancelled.)
It definitely is used by Americans in the airline business and perhaps in other modes of transportation (ship, rail, bus, etc.). I can't speak for the hotel/resort side of the travel industry. "Passengers" wouldn't make sense there, though.... it is not used by Americans at all (including in the travel industry, to the best of my knowledge).