Can I store documents in Word for Mac and associated Endnote references in iCloud so that I can work on them on different devices?

I suggest you use DropBox, OneDrive, Box.com or one of the other free cloud services.

OneDrive recently upped it's free storage to 15GB but if you have O365 you get 1T. https://blog.onedrive.com/new-onedrive-storage-plans/

See this discussion on using iCloud with Word. Can Word documents be stored in iCloud?

Glad I could help.

BTW, if you need to share files larger than 2GB, I believe that DropBox is the only service that allows large files. However most documents are well under that limit so any of the free services should work for you.