Not quite the PDF-viewer in the markdown editor feature, but … I emulate the page using CSS. I.e., in dark mode I create a white-backgrounded, US-letter-sized rendering that turns out to be close to the final result. It doesn't page, of course, but … it's a close approximation. This I do with some far-trickier-than-one-could-imagine CSS in the userstyles.css file.
And then, because I am a writer, I generate manuscript-ish renderings for sharing with critique partners and whatnot. I do this with a whole stack of CSS I pull in from an external source.
An in-app PDF renderer would be a nice-to-have for me. But I wanted to share with folks what is possible … What I used to fill in that need for myself.