I use a pressure washer all the time for putting pipe under sidewalks. Use a tip that gives just enough opening and push the pvc or ads through. Best to pressure down grade so the water and spoils have a place to go. If that isn't the case, dig a hole at the end you will be pushing to and the water and spoils will have a place to go. Your trench leading up to the sidewalk will have to be dug first, so that the pressure washer wand can lay flat and tunnel through with only a slight pitch.
Kirk