Hello,
you have to use either wpdm upload file or select from media. You can not change from one to another after uploading.
Also, by selecting from media, the download link become direct.
If you checked “Open in browser” option in wpdm settings you need to uncheck it to download the file instead of opening in the browser.
If it doesn’t fix the issue, you have to use only the wpdm upload option instead of media library to make the files downloadable.
Hope you understand.
Thanks