In a large bowl, combine orange juice, lime juice, adobo sauce, achiote paste, cumin, oregano, salt, and black pepper. Mix well.

Add the pork shoulder chunks to the marinade, ensuring they are well coated. Cover and refrigerate for at least 1 hour, preferably overnight.

Transfer the marinated pork and any remaining marinade to the slow cooker. Add the pineapple chunks on top.

Cover and cook on low for 8 hours or on high for 4 hours, until the pork is tender and easily shredded.

Once cooked, shred the pork with two forks and mix with the juices in the slow cooker.

Warm the corn tortillas in a skillet or microwave. Fill each tortilla with the shredded pork, and top with cilantro, diced onion, and a squeeze of lime.