Add Duplex entry to viewer preferences dictionary.
Duplex description from PDF Reference:
The paper handling option to use when printing the file from the print dialog. The following values are valid:
Simplex - Print single-sided
DuplexFlipShortEdge - Duplex and flip on the short edge of the sheet
DuplexFlipLongEdge - Duplex and flip on the long edge of the sheet
Default value: none