This is a screengrab of the PDF viewing options in Apple’s Preview app. It’s how you view a PDF on screen: as a single page, one at a time; as two pages side-by-side like a book, or continuous scroll where as you scroll through the pages, the next page slides into view below the current page, in a continuous line of pages.
I can’t help but think the keyboard shortcuts for these can be improved. Consider this:
Single means one.
Two means, well, two.
Continuous doesn’t directly mean three, but it does give the impression of some, or more than a few. When you think ‘continuous’ you think there must be a bunch of things there, because if there wasn’t, the ‘continuous’ aspect would end pretty quickly. Hence, continuous essentially means more in this context.
⌘3 makes sense as shortcut for ‘continuous’ because three is more than one or two.