This is an odd request and my first impression is you should run away from this printer as fast as you can, but I'm willing to give the benefit of the doubt for a short time. How is this getting printed?
When you say you exported to PDF, do you mean as a PDF made of single A5 pages with added bleed and crops, or did you try to export spreads for printing?
Thanks for your reply Peter,
I exported the book as single pages to create a multi-page PDF. They are A5 but the printer wanted them presented on A4 pages?? I thought it was odd too, but I actually just figured out how to do it via Adobe Acrobat Pro - I went to the "Crop pages" option and changed the page size there....
Glad it's solved now anyway.
I'd like to know WHY they had this requirement. It makes no sense to me for imposing, and if they are printing and trimming book blocks rather than folded signatures, they ought to be able to tell the printer how to position a small page on a large sheet, so I'd be very on top of getting proofs and making sure they can actually handle this job without screwing up your work.