what is the best way to travel from portland,oregon to mexico, acapulco?

there are no non-stops from pdx (portland airport) to acapulco.

continental offers 1 connection through houston.

hwy 1

Take Amtrak from Portland to San Diego and fly from there?

Flying, you can take Mexicana air from Portland to Mexico City, and transfer to a plane to Acapulco. If you want a little cheaper you can book through US Airways which would fly you to Phoenix then transfer to Mexicana there and fly to Mexico City then to Acapulco.