Scrolling has switched from horizontal to vertical

I've been using Adobe Digital Editions to read e-books on my MacBook Pro running Catalina and on my iPad Pro. Both had horizontal scrolling, but when I opened it on my Mac today, scrolling was vertical rather than horizontal. I don't see a way to change that back. Horizontal scrolling is preferable because it scrolls one page at a time. Vertical scrolling does not do that.