I've raised this issue before (https://premium.wpmudev.org/forums/topic/i-am-having-download-and-upload-issues-with-documents-on-coursepress-pro) and I see other people are also having the same problem (https://premium.wpmudev.org/forums/topic/coursepress-file-download-error-file-is-empty).
Is there a solution for this yet?
I have done as below, to try and problem solve, but nothing worked:
deactivated all plugins
increased memory both in the config and in the php.ini
tried various different files types (they are fine to download straight from the media library, there is only a problem when using coursepress).
I would like to get this sorted asap and happy to provide access if needed.