How to trim an exterior door to fit new flooring
Take your door off and get a good look at the bottom. There's a fair chance it's an aluminum or steel skin on a wood core. If so, you'll want to get a blade (I'm assuming circular saw is your weapon of choice, here) that's rated for whatever you've got. In addition, you'll want a U-shaped door sweep (well, 3 sides of a box, really) to cover up any nasty metal edges.

Best practice would be to splash a bit of primer on the raw wood you've exposed. 

Depending on the configuration of your door, a new threshold might be a good thing.

At a minimum, wear eye and ear protection... a nuisance dust mask is also a good idea.