Sumatra is great, though occasionally I wish it were possible to have something like Firefox's wrapped scrolling [1] where more than 2 pages can be shown side by side. On a reasonably large monitor, being able to quickly zoom out to see e.g. 30 pages (sometimes all the pages of a journal article) at the same time can be very useful. You can be on p. 20, then go and quickly look up a definition on p.2, then frictionlessly switch back to p. 20. Having to remember a page number to go back to, or a key word to search for, or to worry about where the "back" button might take you (as with most PDF readers) is just too much friction.

[1] https://superuser.com/questions/1365482/how-to-view-a-pdf-wi...