In Acrobat 8 (and 7 as I recall), you can save a background within Acrobat and then reuse it later. If you want to apply the same background to a number of PDFs you can set up a Batch process to apply the background to a number of files and the you'll only have to specify the background settings once. Alternatively I'm sure you could do it in JavaScript but I'm not that up-to-speed with it to give a blow-by-blow list of how to do it.