There’s no reason, in principle, why you can’t have a plugin to display PDF directly within the messaging application - so it is at your fingertips. That may not be good for editing back and forth however.
Seeing as we are digressing quite a bit … you will notice that fancy equations in Wikipedia are done as SVG (for good rendering on the screen) but the SVG still contains the original LaTeX, which may give better a11y behaviour e.g. convert on the client-side into Nemeth Braille (as per the previous post) and ignore the SVG itself.
To me it comes back to: a messaging protocol and application is a transport mechanism; what the recipient does with the message is up to it.