What is Markdown?
Markdown is plain text with a few simple symbols for formatting. A line starting with # is a heading, and one starting with - is a list item.
Why people use it
It opens in any text editor, it's never locked to one app, and it works well with GitHub, Obsidian and most note tools.
Getting a PDF into Markdown
PDFs don't store headings as such, so a converter has to guess from text size. Expect to tidy up tables and columns afterwards.