Trying checking the security settings (Ctrl-L), you are likely Restricted from editing that particular PDF. You can either unlock it if you can, or ask the originating company/source to provide an unrestricted version. You can also just print it to PDF or use any number of various online tools to strip the security, but be careful as you may be doing something you're not supposed to be doing (or even illegal) if you just starting stripping out security restrictions.
