If a restaurant wants you to use your phone and proceeds to serve a website that doesn't work on people's phones, that's on them, full stop. Even if the user can accommodate the brokenness, it's still unacceptable that they should ever have to.

Often the menu is an embedded PDF of uncertain origin that you can't really zoom in on, and font size increases does nothing.

Sure, and with physical menus you can bring along a magnifying glass for when they are printed too small. But then, this sort of things are not considered acceptable in the real world, why should we accept them in restaurant websites?

Does that actually change the font size or does it just zoom?