First choose a directory where you'll keep the files. Example:
mkdir -p /usr/local/overlays
and add it to the PORTDIR_OVERLAY variable in /etc/make.conf:
...
PORTDIR_OVERLAY="/usr/local/portage /usr/local/overlays/pd-overlay"
...
To sync files from the pd-overlay do the following:
cd /usr/local/overlays
svn co https://pd-overlay.svn.sourceforge.net/svnroot/pd-overlay/pd-overlay