Although their holes are often large, they don't typically kill a tree because the damage doesn't encircle the entire circumference of the trunk (i.e., girdling). However, pileated woodpecker feeding holes point to pre-existing damage to the tree, often caused by carpenter ants or beetle infestations. If you look closely inside the hole, you may be able to see the network of tunnels bored by ants or beetles. You can also sometimes look around the base of the tree where the woodpecker was feeding to find their droppings, which will contain the remains of their prey.

That being said, pileated woodpecker holes can create entry points for other tree pests or diseases. You can try applying a tree wound dressing, such as Treekote, to the holes to reduce risk of post-wound infection. 

To prevent further woodpecker damage, you could wrap the trunk with wire fencing or metal sheeting to physically exclude woodpeckers. But if the insects they're feeding on occur broadly up the trunk, then the woodpecker may just start creating holes higher up.  I've heard others using scare tactics such as reflective mirrors or similar objects dangling from branches, but do not know how successful these methods are. Typically wild animals will eventually habituate to these sorts of things once they realize they're not a real threat.

Hopefully the woodpecker completed its feeding on this tree, cleared the pre-existing insect infestation, and will not return.

Hope this helps!

Best,