Making a second, third, or fourth set of manuals would have obtained substantial economies of scale whether in the 20th or 21st centuries.

There was a time where PDF didn’t exist, there were no tools to create, let alone view or print manuals. Even when these became widespread, a number of troglodytes would get bent out of shape if their new appliance didn’t come with a (preferably bound!) paper manual. Most of these folks have either passed on or begrudgingly assimilated.