Thank you both for the answers. I will have a look to the link rjpivonka gave me because I see it working fine there.
1. This is a “Plus” Dropbox account (not business but paid account)
2. we use .pdf and .docx
3. No we paid attention not to add special characters nor spaces. We tried with “test-dropbox.pdf” for example
I tried on a fresh installation WP 5.7, PHP 7.4, no other plugins than Blighty, theme Twenty Twenty and still get redirected to a .zip file.
Would you have a demo website to compare and try to understand?
Thank you
