The credentials (user name/password, etc) collected by elenjo/elenjo+/elenjo JS widgets are stored securely in iOS KeyChain. User names are stored with app data on your iPhone. Credentials collected by a particular widget are only used to log into the web site that the widget supports. The credentials are not used for any other purposes. They are NOT stored off your iPhone except inside your iPhone backups.
The documents (bills/statements/logs/summary, etc) downloaded are stored as Elenjo app data on your iPhone. They are NOT stored off your iPhone except inside your iPhone backups.
To delete the credentials for an account, just delete the account. Please note the documents will also be deleted.
To protect the credentials collected by Elenjo widgets and the documents they downloaded, it is strongly recommended that your iPhone has a passcode and your iPhone backups are encrypted with a password.