The answer is a simple yes if your portable projector has a flexible, wide projection angle design. If your projector doesn’t have a lens assembly with a wide rotation range, you’ll need to improvise some kind of stand of base to change its projection direction. This isn’t ideal, as stability becomes an issue, and your projector could slide or fall mid-movie, which isn’t good for anyone. Of course, placing your projector on its back so the lens faces the ceiling is not advisable. You’ll likely block vents and cause it to overheat, force cables like HDMI to bend under the projector’s weight, and again experience unstable display. You need a projector designed with ceilings in mind, and then the answer to the question is also simple: a yes.