I came here to say Zathura (I’ve been actively using it for the last few days going through K&R C for university) but I see everyone else is saying it too. The “d” key will give you dual pane mode iirc. And what I also do is I use “s” to make the pdf match the window width rather than height and then use Capital H and L to read the top and bottom of the pages, and Capital J and K to go to the next 2 pages.