1. When I was studying advertising materials, our manager found out many interesting things. - нельзя изменить, так как речь о разных объектах
2. He was reading the letter while talking to somebody on the phone.
3. We managed to sell more goods after having reduced the price.
4. Last week my secretary sent you the letter pointing out the prices for our goods.
5. He left after having discussed the terms of delivery.