iPad Acrobat Reader - form fields save as unchangeable PDF

Dears,

I hope somebody will be able to help me and show me a way how to solve my problem.

We are using iPads in our training department and PDFs with fillable forms. At the end of the training and after the coach did all entries he will sign the PDF and save it. So at this point, the PDF must not (!!!) be changeable anymore, the form fields should disappear and the PDF should be an unchangeable PDF. 

I hope a lot that you can show me a way how to save it unchangeable. Just a reminder, it has to be done with the iPad and the free acrobat reader app.

Thanks a lot for your support and help!

Best regards,

Manuel Damerow