Higher acidity is generally a positive feature of good coffees that often shows itself as natural sweetness. Some people find these coffees a little hard on the digestion. Fortunately there are a number of coffees that are either naturally lower acid or are specially prepared to reduce their acidity.
There are two main ways coffee acidity is reduced.
Monsooning involves aging washed and dried green beans in large open air warehouses through the 12 to 16 weeks of the monsoon season. Exposure to the very humid winds has the effect of dramatically lower the acidity level.
The second method of lowering acidity is to process the beans "naturally". This means the beans are dried with at least some the pulp of the cherries still attached. Once dried, the rest of the fruit is mechanically cleaned off the bean. This processing method is common in Brazil and Sumatra.