The message “The document was modified on another device” shows when a document in iCloud Drive is open at the same time on multiple devices (most likely the iPhone/iPad or Mac app) and then changed on one of the other devices.
iCloud downloads the changes made on the other device, but you then have two versions of the document: the local copy with the changes you just made and the copy of the document downloaded through iCloud.
The Windows app will prompt you to save your local copy of the document as a new file. You can then continue to make changes to that new file. The original document remains in iCloud and the incoming changes are saved as a new internal version of that document. You can see and restore these internal document versions through the Flashcard Hero iPhone/iPad and Mac app (see below).
The Mac app will automatically show a message in case there are version conflicts when a deck is opened. To see and restore previous document versions: