Any free and reliable app to display Markdown documents in macOS?
Xcode : if you already use it.
Sublime text:  https://www.sublimetext.com you can also install plugins suitable for you. It is much smaller than Xcode, doesn't exceed 100 MB, despite several packages. 

Install packages using cmd+shift+p and then typing "install package". Then type the name of the package, it would show you the webpage of the developer where you can find the documentation. 
VSCode: with some extensions it can work.