The world’s largest drugmaker plans to combine a cholesterol-lowering drug with another that boosts the “good” type of cholesterol. It’s common knowledge that high cholesterol increases the risks of heart attack. But having too little of a certain type of cholesterol can be hazardous as well, and currently there isn’t much doctors can do, except urge their patients to lose weight and exercise.On Tuesday, Pfizer Inc., the world’s largest drugmaker, outlined its plans to change that by combining its best-selling Lipitor cholesterol-lowering drug with another drug that boosts the “good” type of cholesterol.In early trials, the combination pill significantly lowered bad cholesterol, known as LDL, while raising the desirable HDL that acts as a scavenger, scouring arteries of fatty deposits. The stakes are high for Pfizer and the millions of people who take Lipitor every day.