Place sliced beef in a plastic bag set inside a bowl. For marinade, in a medium bowl combine the picante sauce, vinegar, oil, cilantro, garlic powder, and pepper. Pour marinade over beef. Seal bag; marinate in the refrigerator for at least 4 hours or up to 24 hours, turning bag occasionally.
When ready to serve, drain and discard marinade. Divide beef among pita bread halves. Add tomato, avocado, and feta cheese. Makes 8 servings.