As I'd always understood it, the cabin air is bled from a point in the engine where it has already been significantly compressed by the compressor stages. Like a bicycle pump, this makes the air hot. That's why the aircraft has air conditioning packs to cool the air before it's fed into the cabin. The air conditioning, of course, requires energy to run, thus consuming fuel.
But it would be good if someone who actually knew what they were talking about came and posted the true answer.