Why is the space bar doing a page down in some viewer apps, especially browsers and PDF readers?
For me this seems to be a bit counter-intuitive, and also this behaviour affects games and other interactive HTML5 apps, because if you map some action to the space bar it might also scroll down the page, which is not what you want. Also the arrow keys, page down button and the mouse scrollwheel are usually enough options to scroll down the page.
Are there any historical or usability reasons for this behaviour?
Answer
The historical reason is that that's what the spacebar does in more, the lowest common demoninator (and probably oldest) of text pagers. In more, it makes sense to map the largest key on the keyboard to the most common action: show the next page.
In the glory days of more, you couldn't count on mouse scrollwheels, page down buttons, or sometimes even arrow keys. more doesn't even have a "page up" feature (you can't go back).
Although in the text-based world more has mostly been replaced with more powerful pagers like less (which can move backward), that spacebar convention remains, and has even migrated to the GUI world.
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