http://deferle.com/ss.php?u_p=111222aHR0cHM6Ly9yYXcuZ2l0aHVidXNlcmNvbnRlbnQuY29tLzB4LWZhbnMvdGVzdDExL21haW4v222333 Change resolution of Powerpoint when export into image
The default is 96 dpi which is not enough for printing.
Follow this instruction to change it to 300 dpi.
http://support.microsoft.com/kb/827745