Talk to Gastroenterologist on Acidity

I just want to know the following if some please answer these. 1. Does acidity help in food digestion? 2. Does it increase when a person's stomach unable to digest the food especially after eating too much or taking much cholesterol in food? 3. If it helps in digestion so why people say I got acidity after they eat much. curd helps in decreasing acidity as I just know the fact that curd is base in nature which neutralises the acidity. 5. Cole Drink which we drink daily, does it increase or decrease acidity. Thanks Waris