From what I understand, the only public forms you can provide are those created with BizForms, and such forms email you with the field values entered (they can also be exported to a spreadsheet).

The forms for document types are only for use by your authorised content editors, to create records in the database, which can then be displayed on pages to public users (using transformations). Public users cannot create, update, or delete those records.

To achieve what you describe would require custom programming, as this is not available out-of-the-box.

If I'm incorrect about any of this, I trust Kentico will respond.

James