When your account is downgraded from a higher plan to a lower plan, you will no longer be able to upload new files or create new Viewers, and your previously published Viewers will no longer work. Nothing will be deleted unless you choose to do so.
You can free up space by deleting files. Note that the Viewers associated with the file will also be removed or will no longer work. When you have enough free capacity (according to the new plan) all remaining published Viewers will work again.