Canon S450 User Manual page 206

When you wish to perform background printing, set Backgrounding to On.
3
Close
box
Click in the Close box to close the Chooser. A dialog box may appear with
4
this message:
You have changed your current printer.
Please choose "Page Setup..." in all the open applications.
Simply click OK.
You can now use the S450 for printing.
Print Mode Settings
Using the Macintosh Printer Driver 206