If you use the Word Processing program Pages from Apple, you will need to make that file a Word document to submit to a Dropbox in D2L. 

1. After you have completed all word processing and are ready to submit the assignment, go to the File menu and choose "Export To" and click on "Word..."

2. Click "next" (you do not need to set up additional options)

3. Change the file name if you need to and choose where to save the document, then click "Export." 

4. You'll see this screen as it creates another file that is a Word document. If you need to edit your Pages document and submit to a dropbox again, you'll need to go through the export process each time.