We have developed, and are marketing an entire database-driven system of "Online Food Ordering System", an online food ordering (menu-ordering) system, which also functions as a full-featured restaurant-related website. It is based on WP, PHP, and fully working front-end for restaurant clients, and with back-end for management of menu items, user registrations, orders, etc.
It has an attractive custom home page, has the registration/login end-users, the food menu and online food ordering, shopping cart, table reservation/booking, map and direction, blog, gallery, multiple restaurant branches, order delivery management, etc modules. The payment system of the placed food orders is "Payment on Delivery" at present, but we can add (optional) a Paypal/CC payment system to it if you've a good payment gateway in mind. We can add a SMS/Text module for alerting the manager, of an incoming order if you wish to, which is optional at present. Additionally, we can also customize the system for you to fulfill your client's specific needs, if required. We will need to study what system is in place on your client's site before adding these modules to it. Of course, the frontend is mobile compatible.
We can also recreate it as an HTML site instead of the WP-based site on special request. The system is very affordable for a reseller (per sale basis), and you can add your own margins before reselling it. You may request a demo whenever required. And we are very selective in licensing it to our resellers. The system requires our own service to setup the system on client's hosting though.
Another option is, we can develop the database-driven custom module in PHP, etc., not in plugin form, for your client as we have already developed that important part for the above-mentioned system. As you said that the clients need a very easy to use format for the specific module, we will do our best to create a very suitable module where they can easily authorize/edit/add/modify/delete* the menu items. Please PM me the details if you're interested in going for either option.
Of course, the system or the custom module will have real menu items which are manageable. Other words, it's not a 'pdf menu' system at all!
Thanks!