Preview line and half-page scrolling While two-finger scrolling in Preview is nice, it is not very comfortable when slowly reading very long PDFs.

What I've figured out to this point is that I can use space and ⇧+space to move forward and back a page at a time; and ↓ and ↑ arrows to move a couple pixels down.

Is there any way I can set the arrows to move down a line at a time? (Note that with zoomed in document for reading, it takes 3-4 key presses to move down a line)

Or, any way I can set space to move down only half a page?

Maybe there are some other tips for comfortably reading/scrolling long documents?
I do not think line or half-page scrolling is possible. I'm not even sure that PDFs have, embedded, a concept of "line". I think that Preview tries to detect that for the purposes of selecting text (major improvements on this from 10.5 to 10.6).

You could set up a program (AppleScript, Automator, etc.), that simulates pressing the down arrow a certain number of times. Now, perhaps you click on the app in the Dock or run it via a keyboard shortcut, or even via a Voice Command, and it will scroll the amount you set it to scroll.