AFAIK, you cannot.
Have a read on this
discussion too[
^].
Adding more to it from my side, lets say, somewhow you are able to disable save but then you cannot restrict users to do PRINT SCREEN and grab whatever is visible on screen. And since you cannot restrict it, if someone wants to copy/get content of PDF personally, he/she can have it - thus it defies your effort of not letting it saved! What say?