A sheet of paper with written or printed words on it was the most common way of storing and exchanging data until the computer took over. In the west such papers tended to be portrait-orientation pages about 11 inches tall — a standard that supplies the overall shape and size of general-purpose document icons.
Any pages that needed to be remembered or otherwise noted were often folded down at the corner where it wouldn’t affect the content. This happens to create a more recognizable shape than a plain rectangle, so icons representing text documents often add that, while images seldom do.
(Icon by Susan Kare)