Short answer: Yes, the files are safe to delete - they are basically just backups of previous cluster states. But...
Long answer: The Policy Engine (the part of a Pacemaker cluster which decides what to do when something happens within the cluster) safes backups of the cluster state (configuration and state of resources) to
/var/lib/pengine. These files are not vital (or actually even used) for the operation of the cluster, but might come in handy when investigating failures, because the logs are usually pretty sparse about these things.
So it's wise to keep a few of those around, but on a busy cluster they can clog up the partition after a while. You can limit the number of files kept by pengine by setting the pe-*series-max properties, e.g.
crm configure property pe-error-series-max=1000