https://www.phpbb.com/support/docs/en/3 ... ns_manual/
(linked to 3.2 because the image isn't visible in 3.3)
The permission to download files is what you're looking for.
If the permission result isn't what you extencted/hoped for, the use the Permission mask/trace
to get a clear overview of the settings which define the total permission.
Spelling is freeware, which means you can use it for free.
On the other hand, it is not open source, which means you cannot change it or publish it in a modified form.