What is the ruling on working as a promotional representative for infant formula, knowing that the company requires doctors to receive a financial kickback for promoting it, and that the formula is good, harmless, and inexpensive, and that most mothers do not breastfeed naturally? Is this work considered bribery?

The function of doctors is to treat the sick, not to promote companies. Promoting companies involves many corruptions, so your company is not permitted to resort to it; rather, it must follow legitimate means to promote its products. If the company is prohibited from dealing with doctors who demand money, then as a representative, you are not allowed to pay them anything.