When you’re done with your layout, you have to get it to the printer. Your software will save the book as a PDF file. You’ll want to choose the format that is most compatible so that the POD company can reproduce your book just the way you laid it out. That format is the PDF/X-1a format.
And you absolutely without fail must save the PDF as separate pages, not as page spreads. Why? Well, because the POD company has different printers than you do. They do not print a 6×9 book on 6×9 paper, front and back – they print 32 pages at a time on both sides of one sheet of paper. They are out of order and some are upside down … but then they fold the sheet in a complicated way, and they end up with a bundle of 32 pages in order that are all ready for binding. More to the point, the company will get back to you and tell you to do it again the right way – but this is why they’ll get back to you on that.