preview pdf scrolling (also page up/down)
What seems to be happening is that when we used to set Preview in "single page" as the default, a newly-opened document window was exactly the right size, and you could easily up/down arrow through the pages.   

However, now it instead opens the document with the page ever-so-slightly too large for the window from side to side, so Preview switches into "Oh, you've zoomed in and you'd like to scroll around this ONE page", and so the arrow keys become the scrolling around that ONE page keys.  

Except that since it's vertically the right size, up/down doesn't scroll.  Left/right arrows will, though.

If you slightly enlarge your window (or shrink the page) to include some space all around, you'll see the up and down keys work without the option/alt key.

And this behavior is still screwed up as of 2017-05-29 (OS 10.12.5).