How do I print smaller sized layout centered on letter size paper?

I reformatted my document to size it like a paperback book. When I print it
on letter size paper, it prints out at the top of the page with a lot of
space below. Interestingly, it prints centered horizontally on the letter
size paper, probably because it thinks I'm feeding smaller-narrower custom
size paper into the printer, drawn centered from the paper tray.

I want this paperback book sized document to print centered on letter size
paper, both vertically and horizontally, without first reformatting it to
letter size with larger margins.

Is this possible?

Thanks.