Short answer
HubSpot's KB: you need Super Admin or View permission for files to get a direct link. There's no way to force a download without a server-side script, which the files tool doesn't support. Packaging the file as a .zip prompts a download in most browsers.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Replacing the file keeps the same URL, so shared links keep working.
3. Related
Replace a file and keep its URL.