Hello afshid50,
Thank you for using Apple Support Communities! I understand that you're unable to account for storage space used in iCloud, and you'd also like to downgrade your iCloud storage plan. I would be happy to help explore steps to take next.
You can still Downgrade your iCloud storage even if your used storage exceeds that of the plan you're downgrading to. This could however prevent certain features from working properly such as iCloud backup, iCloud Mail, or Messages in iCloud until you free up storage space. Since you identified documents as utilizing the most storage space, I suggest first ensuring that you've made a copy of the documents following the "iCloud Drive files" section in the resource below:
Archive or make copies of the information you store in iCloud
Once you've done that, follow the "Delete folders or files in iCloud Drive" section here next:
Manage your iCloud storage
If the used iCloud storage is still showing incorrectly, ensure that you've permanently deleted any files or documents from Recently Deleted:
Permanently remove deleted files on iCloud.com
On iCloud.com, you may also want to go to Settings > Restore Files, then delete any restorable files. Let us know what happens from here.
Cheers!