Scrolling up and down a page isn't a GMail-specific shortcut but the browser/OS default. You can use <kbd>Space bar</kbd> to scroll down but there's no equivalent to scroll up (you have to use the <kbd>&uarr;</kbd> or <kbd>Page Up</kbd>) to do that.

If the conversation has multiple long messages, you can use <kbd>N</kbd> & <kbd>P</kbd> to jump to the beginning of the later/earlier message respectively.