I've been running GalliumOS on my Acer C720 for a few days now, and I am really enjoying the experience. One problem that I did uncover is the lack of Page Up and Page Down keys. 

Part of the problem is caused by the C720 being designed to be a Chromebook because its keyboard does not have dedicated Page Up and Page Down keys. Nor does it have Home and End keys. 

ChromeOS provides keyboard shortcuts as replacements, Right Alt + the Up arrow for Page Up, Right Alt + the down arrow for Page Down. However, despite the fact that GalliumOS has a keyboard mapper with a default setting for Chromebooks, the settings don't work. 

Fortunately, I have found a fix to the problem that involves running a set of commands found on this page. The fix does  not enable the Right Alt as the overlay key, but I have found that I can enable the Search key (which is placed where one normally finds Caps Lock) and that works just fine for me. I am just happy to get Page Up and Down back!