PES is Brother's native embroidery format, also used by Babylock and Bernina-XJ machines. The file embeds a PEC subsection containing the actual stitch data. PES is the format most embroidery design marketplaces (Etsy, Embroidery Library) ship with by default because Brother dominates the home/hobbyist market.
How to open
Brother and Babylock home machines (PE Design software, Brother Embroidery Box). Free: Embroidermodder, online PES viewers.