In a simmering pot of water cook the chicken to an internal temperature of 160 degrees using a thermometer. Remove from the water and cool completely in the refrigerator.
Preheat the oven to 350 degrees.
In a bowl whisk together the olive oil, lemon juice, and dijon mustard. Mix in the celery, cucumber, red onion, and olives; season with salt and pepper to taste.
Shred the chicken once its cooled, pulling apart with your hands. Season with salt. Add the shredded chicken to the dressing mixture and mix well.
Warm the pita bread in the oven on a baking sheet for 1 -2 minutes.
Layer the pita with cheese, tomatoes seasoned with salt and pepper, spinach, and the chicken mixture. Fold in half and serve with chips and pickles.