When using Dropbox as your offsite location for download files, you will need to copy the download link provided and paste that into the Source field for your Product Files.
When you get this link from Dropbox, check the URL to see if it ends with dl=0. If it does, you will need to change this to dl=1.
When the URL ends with dl=0, this means the file will open directly in the browser instead of downloading. Changing this to dl=1 will allow the file to actually download.