How to always keep a local copy of documents/folder in iCloud?
If you sign out of iCloud, you will be given the option to keep a copy of your iCloud data on your Mac before signing out. Otherwise, the iCloud data will be removed from your Mac, but will still be available on other devices using iCloud.

The System Preferences has a iCloud pane. Next to the iCloud drive check box on this pane is an Options.. button. Clicking on the button produces a popup pane with an Optimize Mac Storage check box. If you uncheck Optimize Mac Storage, then all iCloud documents will be downloaded to your Mac. (Provided enough space is available.)

Normally, Optimize Mac Storage is checked off. This means the full contents of the iCloud drive will be stored on your Mac if you have enough space. Older Documents will be stored in iCloud when space is needed.