Changing security of a PDF document

[Mauro5FBB]

I have some documents that have Security Method: Password Security, and I would like to remove this security using Adobe Acrobat Pro DC. I have the original password as well.

When I open the document no password is asked for. When I try to change de Security Method in Document Properties I get the message: "You cannot change security on this document because the current document is signed or certified".

It's not signed, it's an empty form.

What am I doing wrong? 

Attached an example pdf form.

[try67 · Accepted solution]

This is a LiveCycle Designer form. Even if you could remove the security policy you won't be able to edit it, unless you have access to this application.