book 16 bit

Hi all,

I am trying to save a book layout from Aperture as tiff 16 bit files:

I tried to save the book as an adobe pdf and to save the pdf as tiff images from Acrobat but i only got 8 bit files

I tried to save "pdf as tiff" from Aperture but i cannot see any options to edit the bit depth (also tried to edit it in automator)

Any ideas? Thank you