Can I move up and down an on-screen page by voice command? Can I move up and down an on-screen page by voice command within Google Now?
According to my knowledge and my researches it seems you can't.
If you want to see all the voice commands you can use check this site below:

https://www.google.it/amp/www.greenbot.com/article/2359684/android/a-list-of-all-the-ok-google-voice-commands.amp.html