Man pages matching name libpnm in section 3:

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