PDFs need: image "links" where you can hover over the link and there'll be a lightbox (or popup? O.o because need zooming buttons) of the image. Useful for lecture notes when they refer to images on say the previous page, where even "view> dual" doesn't work

Also / alternatively, a "view>dual" on pdf readers that let's you choose how the 2-pages-views are split (this working whether you're "view>continuous" or not is optional), e.g. create (2page) from
(1,2) (3,4) ..
OR
(2,3) (4,5) ...

Ubuntu still needs a tabbed pdf reader...