Yes. Just check what sort they are but they are non-poisonous as far as I am aware.
Most of my paddocks have pines in, mature and immature Scots, Hemlock, White Pine, Blue Spruce and the normal Christmas tree types. Never come to any harm.
Some of my grazing includes forestry plantation, great for shelter, the horses may nibble on the trees but in general they leave them alone, too spiky for them.
Some of the herds run in the forest behind the closest paddocks in winter, most of these trees are over 50' now but some offer fab protection, the horses and deer get right in underneath them, the only downside to firs etc is that they are absolutely murderous on rugs !