The thing about usability is that it's both objective and subjective, and one can argue that aesthetics is part of usability. For example, I find writing code much more pleasant with Comic Code font, and I can imagine that there are other people that would hate it.
Ever tried changing the font of a printed document? Or a PDF?
PDF -> Nope.
.doc(x) -> Sure.
Website, OS, apps (including terminal) -> Sure.
Now regarding PDF I might've tried a long time ago when reading some old document (like CIA about MKULTRA). I don't remember if I succeeded. But there are PDF editors out there. I do think it likely screws layout (esp. larger documents), but that can be true for .doc(x) as well.
And I can certainly confirm that changing the font of PDF will almost always result in a unreadable mess. Something about how a PDF doesn't have text "blocks" and instead fixes each character making text reflow almost impossible.
A couple of years ago I went into archives of Dutch newspapers to learn whether and how the famine of hunger in Ukraine (known as Holodomor) was reported back in 1930's. Fuck me, it was hard to read those excerpts. But it is what it is. OCR could've converted the font. The problem is, is the OCR accurate? Like, is my search with keywords having a good SnR, or am I missing out on evidence?
Personally, Times New Roman was likely the reason I did not like Mozilla Thunderbird. I have to look into that.