Pittsburgh and Portland are about 2,160 miles apart by road. Split the difference and you arrive near Denver, the city closest to the halfway point between them. That puts roughly 27 hr 38 min of driving on the Pittsburgh side and 20 hr 34 min on the Portland side — the fairest single meeting point among the cities near the middle.
Over this distance most people will fly rather than drive the whole way. Denver still makes a fair, central place for Pittsburgh and Portland to converge, splitting the travel instead of asking one side to cross the country.
If Denver doesn't have what you're after, Omaha and Minneapolis are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Pittsburgh and Portland.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.