Addressing the last: Ello's descent into UI/UX hell was among the reasons I abandoned it.
The Drive PDF Viewer (default PDF viewer on Android) scroll behaviour isn't perfect, but it's a reasonable compromise with minimal surprises (principle of least astonishment) for the platform.
I quite honestly prefer paginated PDFs in general for online reading, as they follow numerous predictable behaviours, with PocketBook and FBReader my preferred apps for this.
The Internet Archive's online BookReader app is also excellent, in its paginated mode. When set to portrait aspect ratios, it converts to a scrolling mode which I really dislike.
The Drive PDF Viewer (default PDF viewer on Android) scroll behaviour isn't perfect, but it's a reasonable compromise with minimal surprises (principle of least astonishment) for the platform.
I quite honestly prefer paginated PDFs in general for online reading, as they follow numerous predictable behaviours, with PocketBook and FBReader my preferred apps for this.
The Internet Archive's online BookReader app is also excellent, in its paginated mode. When set to portrait aspect ratios, it converts to a scrolling mode which I really dislike.