PGM is the greyscale member of the Netpbm family: a three-line header, then one brightness value per pixel with no compression. It is what image-processing courses, OCR pipelines and computer-vision tools pass around when they want a format with no decoder in the way.
Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a PGM file above and choose afterwards.