One bit per pixel - black or white, nothing in between. The most minimal image format in common use, and still the right one for scanned documents, line art and anything headed into OCR or shape analysis. Photographs do not survive it.

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.