Have you ever wanted to merge several PDF files to create a single PDF file? Well, if you work with electronic documents you probably will. There is a very capable command line tool to carry out this and a range of other PDF related operations, it is called pdftk. You can obtain a copy of pdftk here: http://www.accesspdf.com/pdftk and if you are interested in merging, or taking apart PDF documents, this is the tool that you need.
For example, if you want to concatenate three PDFs into a single PDF file, here is the command:
pdftk A=a.pdf B=b.pdf C=c.pdf output output.pdf
I use pdftk on Linux and Windows via Cygwin on a regular basis.