Man pages matching name libppm in section 3:

libppm.3.en	debian 10 / libnetpbm10-dev
libppm.3.en	debian 10 / libnetpbm9-dev
libppm.3.en	debian 11 / libnetpbm10-dev
libppm.3.en	debian 11 / libnetpbm9-dev