I have a PA court appointed judgment against a previous tenant. He works in Washington DC. Can I garnish his wages

I beleive that you will first have to domesticate the judgment in D.C., that is get the judgment recognized in D.C.  If the judgment was on the merits after a full opportunity to litigate, that should be a simple process.  If the judgment was entered on default, however, it will be more complicated.  You will have to start a lawsuit in D.C. and show that the PA. court had jurisdiction over the judgment debtor.