Unused line at the top
On the iPhone X/XS, there is a thin line at the top-most part of the screen that is visible when viewing PDFs with photos or any non-white page in continuous mode. I think the display area size needs some fixing.
In continuous plus immersive (toolbars hidden) mode, we leave space at the top of the first page (only) so that we can ensure that you can view the top of that page below the iPhone X notch. Otherwise, if there was content near the top margin, the notch would block it. When scrolling to subsequent pages, we take full advantage of the screen and some of the content will scroll behind the notch. If you only have a single page, you won't see this behavior, but even then we need to leave that space at the top of the document so that you can view that top margin.