A Markdown file is plain text. Anything that opens text opens it — the question is what makes it look like a document.
An `.md` file is a text file with a few conventions in it: a hash for a heading, asterisks for emphasis, hyphens for a list. Nothing in the file is binary and nothing is compressed, so a text editor shows you the whole truth of it immediately.
That is also why it looks unfinished. The conventions are instructions for a renderer, and until something renders them you are reading the instructions rather than the document.
On Windows, Notepad opens it and shows the raw text. On a Mac, TextEdit does the same, though it may ask to convert the file first — decline, and it stays plain. On either, VS Code renders a live preview beside the source, which is the closest thing to the finished page without leaving your editor.
Dragging the file onto a browser window does not work the way people expect: the browser shows the raw text or offers to download it, because no browser renders Markdown on its own.
Most phones have no Markdown reader installed and will offer to open the file in a notes or files app, which shows the text as written. On iOS, Files previews it as plain text; on Android, the behaviour depends on which text editor is installed.
Converting it to HTML first is usually quicker than finding a reader, because every phone already has a browser and every browser opens HTML.
A file saved as `notes.md.txt` by a text editor that added its own extension will not be recognised by anything looking for Markdown. Rename it and the problem disappears.
Tables, footnotes and task lists are not in the original Markdown specification, so a reader that shows the pipes and brackets literally is not broken — it implements the core and not the extensions.