If the restaurant doesn't have anything besides a menu, /index.pdf is fine—no web design required; reuse the menu they're printing anyway.
The trade-off is that they'll have to pinch/zoom if they have a small display. It's a minor inconvenience to make the exact information they want available instantly.
> index.pdf won't tend to play nicely with screen readers
The horrible Wix sites most restaurants end up using are likely less accessible than a PDF. The Adobe PDF reader can reflow text.
> also sucks for people on crappy mobile networks
The average wysiwyg site builder produces bundles that are an order of magnitude larger than a PDF menu. Also, the PDF is easier to cache correctly and can be easily saved for offline access.
But you can easily serve a desktop version or a small screen version.
index.pdf won't tend to play nicely with screen readers and also sucks for people on crappy mobile networks, so it's a minor inconvenience for some, and straight up unusable for others