I am using Acrobat 7.0 Pro to create PDFs with signature fields and am saving them without any restrictions. When I look at the properties, all properties say ALLOW, including Signing. (I have also created PDFs without blank signature fields, and they don't function either.)
Yet, when I open the PDF in Reader 8, the properties show DISALLOWED for many of the privileges. There is nothing in the Help nor Knowledgebase under "troubleshooting signing," "troubleshooting signatures," or related searches. The Help reads, if you can't sign a PDF, see the PDF's creator about right. Well, I am the PDF creator, and I am leaving the security open!!