Pruning should be done when a tree is dormant (late winter is ideal for most fruit trees), for a few reasons. First, you can see what you have to deal with, and what you are doing, when a tree has no leaves on it. Second you decrease the risk of spreading diseases (esp for Pears with Fireblight, as it is at its worse in Spring!) Finally pruning cause the next bud below where you cut to become the dominant bud, meaning it will branch out. You can control the way a branch grows by cutting above a bud tha points in a direction you may want a branch to grow. Prune a tree for light penetration, as it makes a tree fruit better, as well as too remove dead, weak, or crossing branches. It is said a pruned tree puts out more growth than a tree that is not pruned. Another thing to look into is branch spreading, which can open a tree up without taking out good branches. Branch spreaders work well for pears as they tent to grow upwards instead of out, but are ideal for any fruit tree you want to open up.