The folder "chm2web" in your GameMaker Studio 2 install folder has the manual.
You can open index.html and read it just fine without GMS2 running (although only the Index tab to the left will work if you just open the HTML file).
Put it on a server, and the Contents and Search tabs will work too.
I could put it online, but I don't know what YoYoGames would think of that.