In a blender, puree the pineapple juice, maple syrup, chiles and adobo sauce, tomato paste, 2 tsp. salt and 1 tsp. pepper. Pour the chile mixture into a large slow cooker. Add the beef, turn to coat in the sauce, then arrange fat side up. Cover and cook on high until fork-tender, about 6 hours.
Transfer the beef to a work surface. Using two forks, shred the beef, discarding any excess fat. In a large bowl, toss the beef with 2 cups of the cooking juices. Let stand until most of the juices are absorbed, about 10 minutes; season with salt and pepper. Make sandwiches with the beef and the buns, drizzling with cooking juices if desired.