Not a valid InDesign Interchange document

Recently switched from a Windows to Mac. Went to open an .indd file in InCopy (via Dropbox on desktop) that a co-worker prepared on his Mac, and the text is missing on most of the pages. When I go to check out one of the blank pages, I get the "Not a valid InDesign Interchange doucment" message. 

The .indd file is not an online-only copy. I simply double-click it from the Mac Finder to open it in InCopy. Never had this issue when I was using a Windows and the same Dropbox account, files, etc.