A very easy way of adding passwords to pdf in RHEL 7 is to use the pdftk package.
You can find the RPM at:
yum install https://www.linuxglobal.com/static/blog/pdftk-2.02-1.el7.x86_64.rpm
Once you have the package installed, adding a password becomes a one liner.
pdftk my-pdf.pdf output protected-pdf.pdf user_pw abc
I love it when things that I think are simple, are simple. I’m sure there is something much more fun going on under the hood but that is for another time.