For me also the biggest issue when reading technical or scientific pdf is a difficulty to quickly go forth and back. What annoys me is that it is simply a software deficiency. I know serval pdf readers that do some things right and combined they would make a perfect reader. I love the way smart office 2 handles zooming (when zooming out you can see up to 9 pages at once and scroll quickly). It should pre-caches pages for a more smooth experience (this is a big issue with all readers, even on high end machines rendering is too slow, I don't get why), but zooming is cool. Unfortunately it lacks all other features as it is simply a reader and you can't make annotations, bookmarks, etc.).
To sum up I am waiting for a pdf reader able to quickly pre-cache all pages and render then quickly when I move my finger to flip pages both ways. Being able to zoom out and see many pages at once (with bookmarks, highlights and annotations visible). E-paper is probably still too slow for it, but any modern tablet should have no problems.