Some more info on the issue:
-- I already went to iCloud.com, and the app "plaintext" is not in the list there
-- I go to iCloud.com and then to iCloud Drive, and plaintext is not there either
-- when I go to System Preferences--> iCloud and then choose "manage" the app plaintext is there, but all I can do is delete documents and data, it seems I cannot access them first
-- finally, I wonder if I borrow an iPhone, insert my SIM card and log in with my apple-Id, would I be able to access my documents and e-mail them to me?
thanks for your help
Luis