pxz to jpeg conversion pretty much describes the export of Pixlr graphics documents (.pxz) to JPEG format, something that is certainly possible with Pixlr. As a free users you are limited to two saves per day, but that should be more that enough to convert single image.
If you have many .pxz files that you need to convert to JPEG and do not want to subscribe to Pixlr, the only remaining option is to extract the contents of .pxz files (ZIPs), find your image in .webp format and convert that to JPEG format.