open returns -1 EMFILE

One of my systems has been running a program to create an archive of images and ran into the following issue as reported by strace:

open("/path/index.pdf",O_WRONLY|O_CREAT|O_TRUNC, 0644)
   = -1 EMFILE (Too many open files) 

Recall that open takes a path and returns a file descriptor (or -1 on error). In this case the fix is to adjust the file descriptor limit by editing /etc/security/limits.conf and doing a "ulimit -n unlimited" so that it uses the hard limit set in said file.