When printing 1/2 sized sets from PDF's the 12x18 printer driver is fussy. There are 2 places you can select the scaling: on the generic dialogue box and within the Canon proprietaryr dialogue box. Be sure to select 50% scaling from the GENERIC dialogue box. And 100% scaling from the CANON PROPRIETARY dialogue box. All 4 combinations were tested:
Generic 100% : Proprietary 50% = prints at 1/2 size but only prints 50% of the desired image
Generic 50% : Proprietary 50% = prints at 1/4 size
GENERIC 50% : Proprietary 100% = PRINTS HALF SIZE SUCCESSFULLY!!
Generic 100% : Proprietary 100% = as expected prints full scale but is only able to print 1/2 of the desired information.