Hi Ofer,
From the description you have provided, it seems to me neither of the two provided Document converters is set up and working correctly on your site.
Go to Site administration > Plugins > Document converters > Manage document converters. I Unoconv or Google drive Enabled?
Go to Unoconv Settings. Is there a green checkmark beside the box for the Path to unoconv document converter? Click the link for Test unoconv path. Is it configured properly? Click the link for Download the converted pdf test file. Are you able to see the PDF?
If you are not using Unoconv, are you using Google Drive instead? For it to work you must have OAuth 2 service working. And, you can then test if it is working properly.
Without one of the two converters being properly set up and working, the problem you describe will continue to exist.