Serene,
Never attempted before reading your post, but just did using exact same software applications and versions except the Adobe 9 is standard not Pro, and I was able to open a document, attach a second page to it, save it and close it. I then went to a different patient, came back, opened it and there was the modified document in all its glory.
I have had issues similar to what you are expressing outside of AC with secured .pdf documents and have seen similar error messages. I suspect that if you were to look at the document security settings: From the menu bar (F9) on the document, select Advanced >> Security>> Show Security Properties, it may show that the file is protected from any number of changes and also shows the document restriction summary list. If it is secured, it would have to be unsecured before you'd be able to add to it. See if this helps.
GDN