PBM is Netpbm's bilevel format - one bit per pixel, pure black or pure white, nothing in between. It is used for scanned documents, fax-era imagery and dithered art, and its packing convention surprises people: a 1 bit means black.
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 PBM file above and choose afterwards.