Prepare the marinade:
In a mixing bowl, whisk together achiote paste, vinegar, orange juice, lime juice, garlic, cumin, paprika, chili powder, oregano, salt, pepper, and olive oil until smooth.
Marinate the pork:
Add the pork chunks to a large resealable bag or bowl. Pour the marinade over the pork, ensuring it is fully coated. Cover and refrigerate for at least 2 hours (overnight for best flavor).
Cook the pork:
Stovetop method: Heat a large skillet over medium-high heat. Cook the pork in batches until browned and cooked through, about 12–15 minutes.
Oven method: Preheat oven to 350°F (175°C). Place marinated pork in a baking dish, cover with foil, and bake for 1.5–2 hours, until tender.
Grill method (authentic style): Skewer marinated pork and grill over medium heat until charred and juicy, then slice thinly.
Warm the tortillas:
Heat corn tortillas in a dry skillet or directly over a gas flame until soft and slightly charred.
Assemble the tacos:
Place a generous scoop of pork onto each tortilla. Top with diced onion, cilantro, salsa, and a squeeze of fresh lime juice. Add pineapple chunks if desired.