PD stands for PortlanD, X may stand for eXtended, but I'm not 100% sure about that letter. PDX can either mean just the airport, or the greater Portland area, depending on the context.
it's short for KPDX which is the international AirNav code for Portland ie a very specific spot on the globe. bigger airports have easy to understand codes like (k)SEA while newer small ones get weird ones like OGG (old/us) and and PHOG (current international standard) are both Kahului Airport maui, hi. When ever you go anyplace by commercial aircraft a flight plan Lists many of these 3, 4 letter codes describe a series of points along the way (some are just a beacon/point on the chart) many become slang for the town the airport is near just as Beantown (Boston) and nola (New Orleans) denote other identifiers. Once you know the ones important to you they make a quick Google shortcut for maps and info nearby KTCM= McChord (now joint base Lewis McChord)= and about a thousand other things doesn't work well, but ABQ= Albuquerque works much better.