I've boiled cider a few times. I did it about three years ago, so I don't remember exactly what I did. I believe I added pectic enzyme prior to the boil. I made a cider by reducing the volume by about half. I had trouble getting it to ferment fully and I ended up with a sweet cider that had an intense cooked apple flavor. It wasn't crystal clear, but not overly cloudy either. It was an interesting experiment, but I've never felt the urge to do it again. If I did it again, I'd probably reduce it by a little less and add a little cinnamon, vanilla, and nutmeg.
If you boil cider long enough, you can make an apple version of maple syrup. I reduced some cider a little too much. Probably about 80%. I put it in the fridge to cool. It hardened as it cooled and ended up so thick that I couldn't even pour it. Apple taffy anyone?