Let’s assume that we have given id of process (PID) 6546 and we want to find out which files process actually uses.
Very important here is actually, because we don’t want all the files used by process so far.

To do it we can use proc pseudo file system.
Under /proc we can find directory for each process actually running on system.
Names of these directories are PIDs of processes.

In each of these directories we can find interesting data about the processes(file descriptors, command line arguments, environment variables).

In directory /proc/6546/fd we can find all files actually opened by process with pid 6546.

Links

http://www.tldp.org/LDP/Linux-Filesystem-Hierarchy/html/proc.html
http://man7.org/linux/man-pages/man5/proc.5.html
https://www.cyberciti.biz/faq/linux-pidof-command-examples-find-pid-of-program/