Mac – refresh Preview from command line

Auto-opening a downloaded file from a browser is done on pretty much a browser-to-browser basis. Since you asked about Chrome, here's the relevant blurb from Google's Docs:

Automatically open certain types of files

If you want certain types of files to always open after they're done downloading, click the arrow next to the file button in the downloads bar and select Always open files of this type. To prevent potentially malicious files from automatically downloading to your computer, this option isn't available for executable filetypes, such as those with .exe, .dll, or .bat extensions (for Windows), and .dmg extensions (for Mac).

Personally, these features weren't visible when I tried it, but then I hardly use Chrome other than for the odd web site test.

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.