You clearly have no concept of embedded operating systems in general. Most can read txt trivially (often in the form of config files and the like) but have no need to understand pdf.

Fact is, PDF is orders of magnitude more complex than txt and needs similarly more complex specialised tools to understand it. That complexity is the problem that makes it inferior.

Plain text is not inferior in literally every way. It's superior in not needing additional tools to understand. Not no tools, but a bare minimum which cannot be said for pdf.

True that formatting and images and other niceties are not in txt. But when the core question is "how to preserve the text" (not "how to preserve this specific prettified version of it"), then txt is superior.

Personally, I currently would advocate for markdown. All the advantages of .txt as it appears on disk, and some basic formatting understandable by parsers without requiring such formatting to be understood