This question was already answered by Allround Automations:

It is unsupported, but if you have an ODBC datasource for your database, and install the Oracle Open Client Adapter (OCA) for ODBC, you can connect to any database

And it still doesn't make much sense.
PostgreSQL's language is PL/pgSQL, not PL/SQL. So, most or all programming code wouldn't run, requiring at least slight changes. You'd be able to run simple DML queries, but that's not the reason to use PL/SQL Developer in the first place.