I personally would never say pull down the window. I would only ever use "roll down the window" when I'm in a car. (It stems from the fact that old car windows were rolled down using those rolling things, as they were not electronic)

I would say 'pull down'. This of course assumes you are operating a sash type window that goes up and down - here in Australia they are not very common, except in older houses. We would't say 'roll down' a window. The only thing we would roll down is a roller shutter (which would be on the outside of the window, but separate from the window itself), or a garage door - we have roll down garage doors, whereas in other places the lift up type seems to be more common. For something to 'roll down' (or up!) it has to wrap around itself (the way a carpet rolls up), otherwise you wouldn't use that term to describe the action of opening and closing.

In the U.S. we use "roll down the window" in cars sometimes, even if it is a newer car with power/electric windows. I would never say "pull down the window." "Roll down the window" is only for cars. Otherwise, you would just say "open the window."