Stir orange juice, lime juice, olive oil, oregano, garlic, cumin, pepper, and salt together in a bowl; pour into a large resealable plastic bag. Add pork shoulder to bag and seal, removing as much air from bag as possible. Massage bag to assure pork is covered in marinade; refrigerate at least 1 hour to overnight. Remove pork from bag and place in a pressure cooker. Pour marinade from bag over the pork. Lock pressure cooker lid in place and set steam vent to Sealing. Select Pressure Cook (Manual) and cook for 50 minutes on High pressure. Once the cooking cycle has completed allow pressure to release naturally for 15 minutes, then set steam vent to Venting to quick-release remaining pressure. Remove pork to a cutting board; break into smaller pieces with a fork. Skim fat from liquid in the cooker and discard. Serve liquid with the pork.