I understand that the button is grayed out until a change is made, but let me point out that Adobe Reader is only a READER.... it doesn't exactly allow for changes to be made using how do I say this... READER.
The save button should work as again this is a READER and not the full version. Addtionally why not allow us to link the SAVE AS to the SAVE button on the tool bar... Less clicks makes everyone happy!!
Reader is capable of changing a document and saving it, depending on the version and what usage rights are present in a document. For example, Reader 10 allows users to add sticky note comment and saves. If enabled, it can also use the Typewriter tool to add text. It can add other types of comments if they are enabled. If enabled with other rights, it can save a filled-in form. With another usage right, it can create new pages based on templates and save the resulting document.