Combine the paprika, achiote, cumin, salt and pepper in a small bowl. Rub the spice mixture all over the pork.
Add the onions, garlic, scallions, beer and ½ cup of water to the slow cooker and whisk to combine. Add the pork, cover and cook on low for 8 hours.
Remove the pork and transfer to a cutting board. Strain the liquid into a saucepan, bring to a boil and cook until reduced by half, about 7 minutes. Season with salt and pepper. Pull the pork using two forks and mix in a bowl with about 1 cup of the reduced cooking liquid. Serve on buns or with arepas.