To make content available offline you can mark folders as offline folders. To do so, select the desired folder in the library and click on “Offline Available”.
A menu window will open:
Here you can make either the single folder or the folder including all its subfolders available offline. With a click on one of the two options, the folder is now made available offline. You can now instantly identify such folders via the green offline symbol.
If a folder is no longer to be available offline, follow the same steps for making a folder available offline and select the third option “Make folder and all subfolders unavailable offline”.