>shouldn't "down a page" be done with PageDown?

PageDown doesn't reliably exist - plenty of laptops put it behind a function-key combo, making the only action you're doing on the page a two-handed affair.

> who came up with binding this behavior to a spacebar as well??

Apparently, it comes from the `more` command[1], because they couldn't reliably assume terminals would have a PageDown key, and because spacebar was the biggest key on the keyboard so it was the obvious choice for the one action in `more`.

[1] https://ux.stackexchange.com/questions/53110/why-does-the-sp...