Having seen several posts related to the following error, this has caused problems for many people however I have discovered at least one cause of the problem and the information below may be helpful to others.
"This document enabled extended features in Adobe Reader. The document has been changed since it was created and use of extended features is no longer available. Please contact the author for the original version of this document."
Note, that when opening a document that contains a digital signtaure, the date and time of the signature is set according to the pc of the person who created the document.
The above error message will be produced if the date on the system which is opening the document is earlier than the signature date.
To correct this problem, simply close the document, update the system date to ensure that it is after the date of the signature and re-open the document. The message should disappear and the extended services will be available as normal.