/usr/share/doc/libpqxx3-dev/README.gz is in libpqxx3-dev 1:3.1.1-0ubuntu4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libpqxx3-dev/../libpqxx-3.1/README.gz
/usr/share/doc/libpqxx3-dev/../libpqxx-3.1/README.gz