If the menu is only available via PDF and the PDF is not tagged, then the website is not considered accessible.
However, if the menu is also part of the main website (ie, the menu is available in html like the rest of the page) and assuming the menu is using semantic html and that the rest of the restaurant site is accessible (huge assumptions), then the website would be considered accessible even with an inaccessible PDF because the menu is available in a "conforming alternative version".
Note that I'm using "conforming alternative version" in the opposite way it is typically used. Usually that phrase is used if your main website is not accessible but you provide an alternative version of the website that is accessible. The alternative version you provide is considered the "conforming alternative version". An alternative version is generally discouraged because it tries to give a "separate but equal" experience for all users. It's best practice to make the main website accessible.
In summary, the best user experience is if the main website is accessible and any PDFs on the site are also accessible. But the website can still be considered accessible even if it provides inaccessible PDFs as long as the info in the PDF is available somewhere else on the site in an accessible format.