Its a very controversial topic in mind of many people, and very common topic to lure money in the name of Pitru Dosh who are facing challenges in different stages of life.
In astrological literature there is no mention of Pitru Dosh but many scholars from past have referred to this dosha. Though it can't be completely denied because astrological literature were not passed always in written format. But to have such dosha's 2-3 members of family must have strong indications and combinations in horoscope. The time period will be similar in each of them.
Let us understand Pitru and Pret dosha.
Pret dosha happens when people who leave their physical body due to many reasons might not be able to transcend to higher lokas. In such case one may be under pret yoni for sometime. When souls are under pret yoni, they try to communicate with family member and if they don't listen they might create troubles for their kins because they want liberation for themselves. Few souls with bad karma get pret yoni and when they are in pret yoni due to samskaras; certain powerful tantriks ask them to perform their malicious work.
There are many cases like this which we have come across. Will discuss those things later. Cutting short, pitru's can't be pret but our ancestors can be pret before moving to pitru loka. So, if any astrologer or anybody tell you that you have pitru dosha its a false claim. Because one can be Pitru only if karmas are purified, all ancestors don't become pitru. We have to perform pujas to liberate them from pret and different yoni's.
One important point; offer food to pandits only during Pitru paksh not every Amavasya. On Amavasya it doesn't go to pitru's. Soul under pret yoni get such food and become more afflicted under maya. If you want to liberate souls under pret yoni, perform mantra and hawan for them.
Hope this will be useful for you