When you open PDF files that are Reader-enabled using Reader Extensions server (version 5 or 6) in Reader 9 or later, you receive the following error:
Reader-enable the PDF files again using a later version of the ReaderExtensions server (that is LiveCycle 7, or ES).
This error occurs because Adobe Reader 9 no longer supports the ubiquity technology provided by ReaderExtensions server version 5 or 6.
Extract from: http://en.wikipedia.org/wiki/Adobe_Acrobat
“Adobe Reader 9 drops support for Adobe Reader Extensions 5 and 6 which permit Adobe Reader client software to save changes to filled-in forms in PDFs. Adobe Reader Extensions 6.1 and newer are still supported. Legacy PDFs are still viewable, however they open with the warning “This document enables Reader capabilities that are no longer enabled in this Reader version.”
This change in discussed in the Reader Extensions upgrade information on adobe.com: