Dynamic/Responsive "ad" for Point of Sale on iPad

What does "put on their iPads" mean? Without more details, it is virtually impossible to guide you further.

A single PDF cannot be responsive. You could make more than one. Really, responsive design needs HTML tools, and forgetting most of the tech details you know about print design... What do you gain with a PDF?

I wouldn't bother with two versions -- just produce a portrait PDF and let users view it on their iPads, and they can decide if they want to view it in landscape format by turning their tablet. Another format worth considering is InDesign's Publish Online, where you can include some effects, such as animations, but the user has to be online to access it. Another option, at extra cost, is the in5 InDesign plugin that produces great HTML5 documents, that can be responsive, so suiting your initial requirement:  https://ajarproductions.com/pages/products/in5/?ref=footer

I suggest you mock up a couple of test pages and show it to some dealers to see how they view and use it.

This may answer your question;

https://indesignsecrets.com/printing-and-exporting-alternate-layouts.php

and here is a course on the subject:

https://www.lynda.com/Digital-Publishing-Suite-tutorials/Creating-alternate-layouts/97963/107301-4.html