Bring a large pot of salted water to a boil. Add the pasta and cook until the pasta is al dente (tender but firm to the bite). Drain the pasta, and rinse with cold water. Transfer the pasta to a large bowl and immediately coat with ¼ of the dressing and toss gently. Combine the pasta with the tomatoes, peppers, olives, onion, spinach, cheese and pepperoni. Pour in another ½ of the dressing and toss gently to incorporate. Cover the mixture and place it in the refrigerator. Allow the mixture to marinate for at least 4 hours. Prior to serving, add in the remaining dressing and toss gently.