Wp.com support page says “Using Scribd is Free” http://en.support.wordpress.com/scribd
But Scribd seems to be free for one month only.
Could someone tell me what happens with the documents (including access to files and settings) when one does not buy the upgrade within that period, please?
You don’t need a paid subscription to Scribd to upload documents there and embed them on WordPress.com. Scribd subscriptions give access to their other services (like their library). You can read more about that on the Scribd support site.