Markdown is plain text with simple typed marks for structure: a heading starts with #, a list starts with a dash. If you can read a normal note, you can read Markdown, and that’s the point: people and AI can both read it without a heavy app in the middle.
Imagine a paper note you mark up for the reader. A title at the top, an underlined section, a short list, a circled word. Markdown is the digital version, with typed characters doing the marking.
Why you care
Agents read, edit, and reuse Markdown cleanly, while a PDF can bury the real text under layout noise (modern models handle many text PDFs, but Markdown stays cleaner). The bigger unlock: Markdown lets you own your knowledge outside any single app. Files locked in a proprietary tool need that tool; Markdown opens almost anywhere: Claude Code, Obsidian, GitHub. A README is usually Markdown, a skill can be one, and a vault is often a folder of Markdown notes with YAML frontmatter adding structured fields at the top. A spreadsheet should stay a spreadsheet; for durable notes, process docs, and agent instructions, Markdown is usually the cleanest format.