Uwe Hermann | A slightly paranoid Debian developer

One of the single most useful packages when it comes to PDFs in Linux is pdfjam.

From the website:

• pdfnup, which allows PDF files to be "n-upped" in roughly the way that psnup does for PostScript files.
• pdfjoin, which concatenates the pages of multiple PDF files together into a single file.
• pdf90, which rotates the pages of one or more PDF files through 90 degrees (anti-clockwise).

The installation is easy as always: apt-get install pdfjam

PDF is not exactly the most easily editable format out there, but these tools can save you lots of time and trouble. Just recently I needed to merge two PDFs into one (and I didn't have any source format of the files). A simple pdfjoin foo1.pdf foo2.pdf --outfile bar.pdf does the job in a few seconds.

Equally useful when you need to print huge documents is pdfnup --nup 2x2 foo.pdf, which sticks four PDF pages into one (thus drastically reducing the amount of pages you have to print)...

Update 2006-09-20: As was noted by several people, pdftk is very cool, too. It can do some other things such as split PDFs, encrypt/decrypt them, manipulate metadata and more...