Open Publisher and print to the Adobe PDF printer. If there is an Acrobat menu in Publisher, you can try that. If the last part does not exist, then you cannot open a Publisher file in Acrobat. According to the compatibility chart, AA 8 and Publisher in Office 2003 should be compatible. You can try a repair of Acrobat and also do updates (at about 8.3.5 or so). You might also have to enable Acrobat in Publisher.
Opening in Acrobat requires PDF Maker (the Acrobat menu) to work in Publisher. This is where enabling may be necessary. Using the printer is always a backup.