My mobile phone company decided, 7 months ago, to save the world by not sending paper invoices any more. Unfortunately they didn't tell me (and registering to see invoices online required jumping through lots of hoops, while holding a PIN number I didn't know).
All sorted now, and I only lost one invoice (they only keep the last six online). So I downloaded six shiny new PDF files. Like my phone company I also want to save the world, so wanted to print them four to a page.
The solution is pdftk (in Ubuntu's package list; also available for other major distributions I believe). This simple command:
pdftk 2008*.pdf cat output all.pdf
It just worked. Nice. It also does lots of other cool stuff, like adding or removing encryption, remove just one page out of the middle of a PDF, rotate, extract text. No mention of i18n, so I don't know (yet) how it will cope with Japanese PDFs. Or PDFs where the text is actually an image. The projects home page is http://www.accesspdf.com/pdftk/
It appears there are also Windows and Mac versions.