What opens a .md file on a Mac?
A .md file is a plain-text document written in Markdown. A text editor lets you inspect or change the source. A Markdown preview turns headings, tables and code into a readable page. Choose the view for the task, rather than changing the file's extension.
For one quick read, open the free browser reader. For a set of documents with local links, keep the folder together and use a compatible desktop reader. Aero Markdown for Mac is in development; download and checkout are not available yet.
Open the source with an app you already have
In Finder, select the file, open its context menu, then choose Open With and a text editor such as TextEdit. Opening plain text lets you check the content even when you do not have a Markdown reader installed.
A heading such as ## Acceptance criteria will still include its Markdown characters in a source view. Seeing those characters does not mean the file is broken. Avoid saving a plain-text source as rich text if you want it to remain Markdown.
Read a formatted page in your browser
- Open the online reader.
- Choose Open .md file, or paste the text into its source pane.
- Read the preview, use its outline, and search for a term.
- Download a Markdown copy if you edit anything you want to keep.
The document is processed in that browser tab. Images are not loaded; math and Mermaid stay as source in this web version. A local link to another document is not the same as a website link: download and open the containing folder in a suitable app to follow it.
Try a document with some context
Download the Project Reading Kit. Its six Markdown files describe a fictional field-notes project: overview, specification, architecture, implementation plan, decisions and release notes. There are no executable scripts or installer.
Start at README.md. The specification tells you what is proposed. The architecture explains the parts, and the decision log records trade-offs. A rendered page helps you read one file; opening the folder helps you understand how those files relate.
Open the kit in Aero Markdown
Choose File → Open File… or Command-O for one file. Choose File → Open Folder… or Command-Shift-O for the surrounding documents. These are separate operations: a file is enough for an isolated read; the folder supplies the file tree and context.
Opening and rendering do not rewrite the source. Editing and choosing Save writes that document. See opening a folder and following relative links.
How do I choose the default app for .md files?
Once a reader is installed, select a .md file in Finder, choose Get Info, expand Open with, choose the app, then Change All. This controls which app opens a file on double-click. It is separate from enabling a Quick Look extension for a Space-bar preview.
Apple documents the default-app procedure. For previsualisation, read Markdown in Finder.
If the document still looks wrong
Check whether you are viewing source or rendered content. Confirm the file contains actual Markdown, rather than a PDF renamed .md. Keep local images and supporting documents alongside their original paths. For a missing link, inspect the filename and anchor before assuming the reader has lost the document.