Air carrier on vectors with PDX TRACON was issued descent to 3000 with confirmation read back; and then instructed to maintain 5000.

Narrative

While on the Moxie 5 arrival to PDX descending to 7000' in VMC; Portland approach issued a clearance of fly heading 330 maintain 3000'. The Captain; who was the pilot monitoring; read back fly heading 330 and maintain 3000'. I confirmed the altitude as per procedure and turned to 330. We were on a right downwind in VMC descending through 4300' when Portland approach said to maintain 5000'. I arrested our descent at 4100' and climbed to 5000'. The captain responded that we heard and read back 3000'. We received no response from the controller and were handed off to the next frequency where upon contact he/she cleared us to 3000'. I believe the Portland approach controller issued a clearance to 3000' in error. During our approach briefing we had discussed the MSA of 3600'. When I heard the maintain 3000' clearance I thought to myself that it was lower than we are usually cleared to on downwind. Since we were day VMC with the airport in sight I thought that we must be number one on arrival and that they were going to vector us for a tight visual. If something seems abnormal or doesn't look right; an additional verification or query would provide additional barriers.