Enabling usage rights in Adobe Reader corrupts PDF text

Hi,

I've created a registration form using Open Office writer which has editable fields. I convert this to a PDF and then add a signature field in Acrobat 8.

At this point everything works fine, all the editable fields etc convert across, but I want people to be able to sign and save the form in Reader, so I check the "Enable Usage Rights in Adobe Reader" feature and save it. Now when I open the PDF in either Acrobat or Reader all the text is completely corrupted and illegible (see image).

Any idea what's going on here?

Thank you

Paul