Following PXF installation instructions (version 6.8.0), there is a missing file even though pxf cluster register reports success.
$ pxf -v
PXF version 6.8.0
$ pxf cluster register
Installing PXF extension on coordinator host and 2 segment hosts...
PXF extension has been installed on 3 out of 3 hosts
$ psql
psql (9.4.26)
Type "help" for help.
gpadmin=# create extension pxf;
ERROR: could not open extension control file "/usr/local/6.25.3/greenplum-db-6.25.3/share/postgresql/extension/pxf.control": No such file or directory