An .md file is a Markdown document: plain text with simple marks for headings, lists, links and other formatting. You can inspect the text in a text editor or read the formatted result in an app such as Zizeg.
Ordinary text, made easier to read.
Recognize the marks. A # starts a heading, ** can mark bold text, and a hyphen starts a list. The source remains readable text.
Read it as a document. Zizeg’s Preview displays headings, lists and tables without making you read all the markers. Opening and reading are free.
Keep the plain file. Markdown is used for READMEs, notes, plans and reports. The .markdown extension uses the same idea; no proprietary library is required.
Markdown source becomes headings, lists and tables in Preview.
Try it with your Markdown.
Open a familiar file and see both views in Zizeg.
Reading is free. Editing and export require unlocking.