Where PPM came from
PPM is part of the Netpbm family that emerged from Unix in the late 1980s, designed to be trivially easy to read and write — the header is human-readable ASCII. That simplicity keeps it alive in image-processing research, computer-vision pipelines and command-line tooling, where being parseable in ten lines of code beats being small.
The technical picture
Compression: None. Transparency: Not supported. Animation: Not supported. Colour: 8-bit and 16-bit per channel. Typical file size relative to WebP: enormous — pixels stored as plain text or raw bytes.
Where it opens
Unix image tooling and academic pipelines. This is usually the deciding factor when choosing between PPM and WebP — WebP renders in every browser but is uneven in desktop software, while PPM has a very different support profile.
Converting PPM to WebP
PPM stores pixels with no compression whatsoever. Converting to WebP typically removes well over 95% of the file size. PPM has no alpha channel, so transparency is lost in that direction.
Converting WebP to PPM
Rarely needed outside Unix image-processing pipelines.. Expect the PPM output to be enormous — pixels stored as plain text or raw bytes — that difference is the price of getting a format the destination will actually accept.