Man pages matching name libpgm in section 3:

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