I've saved forms this way before because all the users of our forms only have the free Reader version. For some reason it now won't allow me to save with the extended features. When I save as and hover over "Reader Extended PDF" it shows my 3 options but none of them are clickable. I don't know why it has saved before, but it won't now.
I also have a secondary issue. A form I previously, successfully saved this way is now not working for one of the users. It won't allow her to save the information she inputs. To my knowledge it has not been changed and it has only been used by a few people. Why would it suddenly stop allowing her to save?
I am working on a Mac, but the users are all on PC's. I've never had this be an issue before though.
Be sure you're not in form editing mode when you attempt to enable the document.
Regarding not being able to save with Reader after initially being able to, this has been reported before many times but the casue is often elusive. The document has somehow changed in a way that prevents the usage rights from being validated. This is not supposed to happen, but it does.