But nobody cares, they do it anyway. Or they did when making paper copies was more common. In practice we just read a PDF on a screen.

Plus the copiers are a bit more clever than you give them credit for. Instead of scaling the whole page down they put a bounding box around the actual content on the pages, to exclude the margins, and then scale that down to fit. The size of the margins changes slightly but not enough that anybody cares. This is especially true when you’re copying a bound book because there’s a really big margin in the middle to accommodate the binding. You don’t need that any more once you’ve printed the pages on a single sheet of paper.