Unable to view "Tools>Protect" to allow me to add a password to a pdf file. No "Protect" option when I open the PDF file. Can anyone assist?

When I open the PDF file, there is not ability to get to "Tools" on one system but when I go to another system I can see it.  But in neither case can I see the "Protect" option to password protectr the file.  Probably very simple but I can't make it happen.  Easy answer for someone but not me.  Please assist.