PDF has two passwords: "Open" password and "Permissions" password. Either will open the document, but you must have "Permissions" password in order to change permissions/password. I think you have just "Open" password which allows you to open password but not to change permissions. You need contact the document's author and get "Permissions" password, provided that the author is willing to do that.
Thank you! That makes sense... I didn't realize there were two different passwords. I will contact the author.