You can (and should) add alt text to other documents besides HTML, including Word documents, PowerPoint presentations, and more. Microsoft has put together a resource on how to add alt text to files made using Microsoft products.

https://support.microsoft.com/en-us/office/everything-you-need-to-know-to-write-effective-alt-text-df98f884-ca3d-456c-807b-1a1fa82f5dc2