File this one in the “interesting to know and possibly useful” category. TextEdit, at least in 10.4, has the ability to combine multiple multi-page PDFs into one document. How? It’s actually amazingly simple: just drag each PDF from the Finder into a blank TextEdit document. Note that for this to work, the document must be in Rich Text mode (Format -> Make Rich Text). It doesn’t matter if the PDFs are single- or multi-page; you can mix and match as you wish.
Single page PDFs appear as you would expect, as a page of text and/or images in the document. When you drop in a multi-page PDF, though, it appears with its own scroll bar, so you can scroll through the PDF within the larger TextEdit window. If you wish, you can add some line breaks between PDFs, just to separate them all a bit more.
As an example, here’s a little movie I made showing a TextEdit document that contains three separate multi-page PDFS:
Once you’ve assembled your mega-PDF, just use File -> Save to store it permanently on your hard drive.
As for the larger question of why you might want to do this, well, I’ll leave that mostly up to your own imagination. Perhaps you’ve got a book that came as 12 separate PDFs, one per chapter? If so, this would be a good way to store them all in one document—as long as you don’t want to print them. That’s right; about the only thing you can’t do with your mega-PDF is print it. If you try, you’ll find you only get the first page of the embedded multi-page PDF.