If you mean the default program used to open PDF-files, the open Windows (NOT Internet) Explorer. Go to Tools -> Folder Options. Then under File Types tab go to PDF extension and Change the Opens with to whatever you'd like it to open with. If you mean Acrobat is the default printer, go to Start -> Printers and Faxes. Select the printer you'd like to be the default, right-click and select Set as Default Printer.