In a medium mixing bowl, combine pineapple juice, soy sauce, olive oil, and brown sugar. Stir until the sugar starts to dissolve and the mixture looks slightly thick and glossy. Add minced garlic, salt, and black pepper, and mix well.

Place the chicken breasts in a shallow dish or a resealable bag. Pour the marinade over the chicken, ensuring all pieces are evenly coated. Turn the chicken a few times to fully cover it in the marinade.

Cover the dish or seal the bag and refrigerate for at least 30 minutes. For stronger flavor and more tender chicken, marinate for up to 2 hours.

Preheat the grill to medium heat. Lightly oil the grates using a brush or cloth to prevent sticking. Allow the grill to heat fully before placing the chicken.

Remove the chicken from the marinade and allow excess marinade to drip off. Place the chicken on the grill with space between each piece.

Cook the chicken for 6 to 7 minutes on one side without moving it too much. Flip using tongs and cook for another 6 to 7 minutes, or until the internal temperature reaches 165°F or 74°C.

While the chicken is cooking, place pineapple slices on the grill. Cook each side for 2 to 3 minutes until grill marks appear and the pineapple becomes lightly caramelized.

Remove the chicken from the grill and let it rest for 5 minutes. This helps keep the juices inside.

Serve the grilled chicken topped with pineapple slices. Spoon any remaining juices over the top if desired.