If the goal is to allow a normal user, "admin" -- to edit /etc/pf.conf and issue pfctl commands, then neither sudo nor su are required. Instead:
# chown admin /etc/pf.conf /dev/pf
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There are many ways to accomplish any mission. The appropriate question is not, "How do I use X?" but rather, "I need to get something done. What are some of the ways I can do it?"