Where is PID file in Linux?

You’ll usually find the PID files for daemonized processes in /var/run/ on Redhat/CentOS-style systems. Short of that, you can always look in the process init script. For instance, the SSH daemon is started with the script in /etc/init.

Can I delete PID file?

PID files should never be deleted, unless you have uninstalled the software that had created them. A PID file is primarily a safe-guard against simultaneous execution. Deleting the PID file voids that purpose for good.