Download and install the offline help from the link above for your Operating System.
Make a desktop shortcut to the manual:
Windows - ...\Program Files (x86)\Image-Line\FL Studio 20\Help\index_offline.html (Right-click > Create shortcut, Move shortcut to desktop)
macOS - .../Users/[your computer account]/Documents/Image-Line/FL Studio 20/Help/index_offline.html (Right-click > Make alias, Move alias to desktop)
NOTE: You can also copy the 'Help' folder to any device that can show web-pages, then browse it in your preferred Web Browser by opening the index_offline.html file.