Why HTML over Markdown? There is no semantic benefit here. In fact Markdown has much clearer ways to define metadata like post date, user, community, URL etc (the front-matter at the top of the document)

JSON is sort of human-readable, and it’s a decent alternative.

Markdown is a plain text format that can render to HTML, but that is a lot easier to read and edit. It’s widely known, used and supported. I run multiple websites that are Markdown files rendered into HTML templates. Most static site generators work that way.

PDF makes no sense here as we’re talking about storing a few lines of text plus a few lines of metadata. It would make the files difficult to read on small screens, and very hard to read by machines.