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.
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.