As I recall, Fireworks' PDF output doesn't respect the print resolution PPI setting. It looks at the total number of pixels, and prints at 72ppi.
With that in mind, you may want to create a copy of the document at a lower resolution (72ppi) and print from that, to obtain the correct size. It won't have the resolution you want though. Alternatively, perhaps there's a method outside of Fireworks for modifying the PPI of the original PDF to obtain the correct size and the full resolution.
Another strategy would be to export pages as full-resolution PNGs, and then assemble them into a PDF outside of Fireworks.
Thanks for your quick reply.
I really need it to be at least 300dpi so I think I'll go with the final option.
Would you know any software (on Mac even better ) like this?