I've been searching an googling but I haven't been able to find a way to create and use layers in iOS. Is it possible? I'd like to overlay my PDF with a transparent layer I can write on, then toggle the layer on and off. If that isn't possible, is there another feature that basically accomplishes the same thing? Like being able to toggle colors of marks on and off? Like, mark it up in different colors, then toggle the marks on and off by color?