>Every OS easily natively reads plaintext. It's a pain to read a lot of it, but it can be done, and manually marked up for better readability and functionality.

The Operating System is not what you're going to use to read a text file. The operating system will run an application that can read text files, but the operating system itself doesn't do that. And if there are any operating systems which are incapable of opening a pdf, then really that's on them for being poorly designed. Every Linux distro I've seen has been more than capable of launching programs that can read pdfs, and I'm struggling to think of how an OS that couldn't even launch a modern web browser would be useful for literally anything outside of incredibly specialized uses.

CPU/Ram aren't even a concern, you might experience slightly longer loading times when trying to pull up the document, but ultimately that's a non-issue that is more than offset by how much easier a pdf is to read.

Additionally, if you tried to use an AI to reformat your .txt file into something more legible, you would have to save it as something other than a .txt, because a .txt by itself cannot store anything but plaintext. And if you do that you're right back at square 1 with plaintext no longer being the ideal format. Also it would require a significant amount of processing power to run the AI.