What is PBM?
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.
What is WEBP?
WebP is Google's image format for the web, and the rare one that does both jobs: lossy compression that beats JPEG at the same quality, lossless compression that beats PNG, and an alpha channel either way. Every current browser reads it; older software often still does not.