1. When I was studying advertising materials, our manager found out many interesting things. - impossible, different subjects
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. - it's a gerund actually
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. - it's a gerund actually