I need to restrict a PDF document from being saved as another PDF document. I have restrictions on the PDF already, but people can still do a save as on the PDF and rename the document to a new name.
You can't do that.
What you can do, however, is add a script to the file that checks its current file-name when it is opened. If the name is not what you want it to be, you can show an error message to the user and close the file, forcing them to rename it in order to be able to open it.
Though bear in mind that people can turn of scripts, or open in software that ignores scripts. Really, you can't control this stuff.