A friend of mine had to email a multi-page document that they had scanned in. Rather than send it as separate files, they were trying to create one PDF file to attach to the email.
But Adobe charge a lot of money (currently £316.25 inc. VAT) for such functionality. Ow.
Enter pdf toolkit, a cross-platform command line program that enables you to manipulate PDF files in a number of ways.
For my purposes, all I had to do was install pdftk using Synaptic Package Manager on Ubuntu, and then utter the totally reasonable incantation on the command line:
pdftk 1.pdf 2.pdf 3.pdf cat output 123.pdf
From problem to solution in about 10 minutes. Thank you, Sid Steward. Thank you Open Source software :)