Note that it is the side (edge) of the page on the *reader’s* right hand. (In some languages (perhaps Thai, but I have no idea), they would consider left and right from the point of view of the book. In English, however, this is not the case.

"the right hand edge of the page" Generally it means the far right of a page. It can be anywhere in a region from top to bottom, as long at it is an inch or less from the absolute right hand edge of the paper. Look at a book as an example. On each page the words end a little before the absolute edge of the page. This blank area can be be considered the right hand edge. There is no specific rule where this "right hand edge" commences, it is just assumed very close to the absolute edge of the page.

can you give more context on when this statement is used? I'd love to help if I can! :)