Upvote:4
Most US car-rental companies ask for your flight number when you reserve a car, and if that flight is delayed, they will know it. The chances that your domestic flight will arrive over an hour late are quite small, and I think if you are reassured by the car rental agency that they won't close up if you are delayed, you'll be fine. I think also many airports have curfews that make arriving between 2 and 6 unlikely - if your flight into PDX is delayed too much, it won't even leave and you'll be arriving in the morning. I wouldn't hesitate to book a car under the circumstances you have here. But do book it - don't arrive without a reservation.
Upvote:6
After some calling around, I confirmed that the rental car companies Avis, Dollar and Thrifty at PDX are open 24 hours.