/usr/lib/libpgm.so is in libpgm-dev 5.1.118-1~dfsg-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/libpgm-5.1.so.0.0.118
/usr/lib/libpgm-5.1.so.0.0.118