Depends on context:
- The top or bottom of pages: Refers to the printed page as the reader will view it. In the layout, a solid bar along the page head indicates its orientation. The top and bottom of the front and back pages on the two sides of a sheet normally match. In some cases, such as for calendars, a head-to-foot alignment is used, so that the top of the front backs up to the bottom of the back page.
The top or bottom of webs: Refers to the surfaces of the paper in web roll