Hi,
You may be choosing the Select From Media Library option. By default, it will do so. You can also choose the other option too. When you attach files from media library as URL WPDM simply redirect to that URL when a user clicks on the download button, but when you upload a file directly or attach a file from server as server path, WPDM handles the download process and so there it can decide whether the file will open in browser or force download. So, if your file is already in the media library, and you want them to download instead of open in browser, please attach those files using the server file browser instead of the medial library file selector.
Thanks.
.
