To think that prior to 2005 there wasn't any page up / page down functionality. After all, that's when Microsoft filed a patent application
for "Method and system for navigating paginated content in page-based increments."
To be exact, March 4, 2005 was the date of the application, and the patent was granted on Tuesday. And it's:
A method and system in a document viewer for scrolling a substantially exact increment in a document, such as one page, regardless of whether the zoom is such that some, all or one page is currently being viewed. In one implementation, pressing a Page Down or Page Up keyboard key/button allows a user to begin at any starting vertical location within a page, and navigate to that same location on the next or previous page.
Shoot, we can't imagine how this works. And we're trying to remember when we saw the first Page Up and Page Down keys on a keyboard. We suppose this is almost as silly as trying to trademark "cloud computing" (as Dell attempted to do).