I have no idea what you're talking about. Are you sure it want a "signature" and not "password"? If it asks you for password then the document is encrypted and you need to know the password toopen it. The fact that it is a "public" PDF, whatever you mean by "public" does not mean anything. If it is not that then:
Which product are you using, which version (including minor), which platform (Win/Mac), which OS/version? Please, include screenshots of the alerts, warnings that you get.