Signs can be enchanted like just like every other enchantable item.
You can get all enchantments you can get on swords except for Unbreaking (Cause signs don’t break)-
Zombiepigman have a 1/100 chance of spawning with an enchanted sign in hand.
The sign is randomly enchanted with 20-30 levels. There is a chance of them dropping their sign.
Oh. By the way: Try not to place them down