How should you go about pruning the pear tree so as not to cause permanent damage?
Basically, you only want to cut limbs small enough to heal over in one season, if possible. This will vary by the age, rootstock, and overall vigor of your particular tree.
Pear trees show very strong apical dominance, and will naturally grow rather tall and narrow, not the best for fruiting. My favorite pruning method for a standard pear tree is a cross between central leader and open center, called a 'modified central leader'.
Basically, central leader has the benefit of being the strongest form, but gets quite tall, and doesn't let as much sun reach the center. Open center is great for some fruit trees, like peaches and apricots, because it lets in more sun, for better ripening, but is often not strong enough for a full crop of pears, as well as being hard to train pears into.
Modified central leader is a compromise. The first part of the tree is grown like a central leader (christmas tree type) tree, then the leader is trained out, and the center opened up into an open center style (to let in light, and maintain manageable height), higher up on the tree.
Start with the basics. Remove all dead branches, and crossed (rubbing)/broken growth. Always cut back to a branchlet or crotch on a larger limb (over 1" diameter), and back to an outward facing bud/twig on smaller growth. Do not leave a stub.
Thin the tree out a little. Pear trees don't like a hard pruning very well, so a light pruning yearly is better than a heavy pruning every few years. When thinning, favor branches that point outward from the center, removing ingrowth/upgrowth. Favor wider crotches rather than narrow ones, where applicable. Try not to remove more than 1/6 - 1/4 of the growth this way. Avoid making cuts bigger than 2" in diameter, if possible.
Head it back. You want an even height. You will be cutting back the tallest branches a little, not more than you need to, just to get the tree down a bit. Remove the tallest stems, down to an outward facing lateral. Use your judgement, don't cut one back super far, or you will end up with a 'witches broom' head of waterspouts. A good rule of thumb is to cut back to a branch that is at least 1/3 of the diameter of the branch you're cutting at that point.
Don't use a pruning sealer/wound dressing. That can do more harm than good. Use a sharp, clean pruning saw for the large cuts (over 1" diameter). The easiest kind to use is a curved hand pruning saw that cuts on pull. You can use loppers for stuff smaller than 1" diameter, and for light stuff (around 1/4" and less), use sharp, tight, hand pruners. You don't want to leave ragged strings.
Try to prune it once a year, lightly, to keep it in shape. One thing to keep in mind is that pears bear fruit on spurs (which are described here). They are stubby, slow growth that forms on well matured wood. They require mature wood and slow growth to grow, and as pruning promotes new growth, be minimalistic once your tree is in shape. This pertains to pear trees. This is not true of others (like peaches and apricots).
What time of year should I prune it?
Prune in early spring, after it starts warming up, right before it begins growing. If you see buds swelling slightly, it would be a good time. If you wait until it starts growing, you will damage a lot of tender new growth. If you wait until summer, your view will be greatly obstructed by leaves/fruit.
Fall/winter pruning can work, but I avoid it, as it leaves the wound open and drying for a long time, increasing the chances of pathogens finding their way in.
Are there any drawbacks from pruning?
Yes, but they are greatly outweighed by the pros.
- Pruning (especially on a neglected tree) causes regrowth, which must be taken care of.
- It is time consuming, and one tree can take hours to do a great pruning job on.
- You are creating lots of wounds, which are open to infection. Pruning back to areas that can heal quickly (not leaving stubs), and avoiding large diameter cuts where possible will help.