Definition from wikipedia:
The ICAO (/ˌaɪˌkeɪˈoʊ/, eye-KAY-oh) airport code or location indicator is a four-letter code designating aerodromes around the world. These codes are defined by the International Civil Aviation Organization, and published in ICAO Document 7910: Location Indicators are used by air traffic control and airline operations such as flight planning.
This is a map of every ICAO airport code wordwide.
Alaska has PA, PF, PO and PP as airport codes. P = Eastern North Pacific.
PA=USA (Alaska) (also PF, PO and PP)
There is no special meaning behind the letter P and these codes, it's just a classification!!