Sharing folders to users with viewer still permits downloads

Direct folder sharing, such as via /2/sharing/add_folder_member, doesn't support disabling downloads.

To disable downloads, you could use shared links instead, which is available via /2/sharing/create_shared_link_with_settings on the API (click on 'SharedLinkSettings' to expand the documentation to see the settings fields). However that doesn't handle distribution/access control to specific email addresses, so you'd need to handle that on your side if you use that instead.