As the sibling comments illustrate, “hidden text” isn’t well-defined, and it has legitimate purposes that end users consciously make use of. The AI needs access to it, for one because the user might actually want the AI to perform actions on the hidden text (not in the sense of following instructions stated in the hidden text, but in the sense of manipulating the hidden text as part of the document), and also because otherwise it might cause breakage in the document if the AI doesn’t consider the presence of the hidden text when manipulating the document.

What AI tools really need is reliable power-user levels of awareness about Word features, and corresponding structured access.

It’s the good old white text on white background. Not really a way to defend against this, except having a no-style or high contrast mode that people actually use. Maybe some warning that would trigger if text is too small, off page or has very low contrast would help?

Headers, footers, notes, comments, alt text, probably dozen of other features. Documents often are lot more than just markdown so properly to support everything you do have a lot of ways to hide text for various use cases.

It's not really important whether the text is visible, you can just put the instructions in the middle of a long document, or use a low contrast color, or a small font size, or any other of the myriad of ways to make text hard to read.

Because, in the 1990s, you would print out your document before giving it to someone else to read. In those times, sometimes you'd want to include text in the document that shows while you're editing it (e.g. notes to yourself or draft text you might want to refer to later) but not when printed.

I believe you would see hidden text by default (but this was a long time ago and I may have misremembered) when in "normal mode" (later "draft mode" and now removed entirely), which was the default view and showed a long continuous stream of text without the computation expense of calculating page break locations. But when you switch to "print layout mode" (now the usual view unless you're in reading mode) it would be hidden, so you could see what the document would be like printed, unless you explicitly turned on the display of hidden text in that mode.

The LLM is reading the bytes of the file, not looking at a picture of its rendering. File metadata exists as well, and change history. Tons of places to hide text.

Even if you processed it via a screenshot, image files are processed byte by byte as well and can contain textual metadata.

The same reason to why you let AI have access to your filesystem. Idiocy, you need to teach AI to be smart somehow.

You train a monkey to learn from a bunch of lower level intelligence monkeys. The same applies for AI. Just this time we are the monkeys.