IF you have several PDF documents, say scans, you can easily merge them into one PDF file using handy pdftk tool:

$ ls
1.pdf  2.pdf  3.pdf
$ pdftk 1.pdf 2.pdf 3.pdf output joined.pdf
$ ls
1.pdf  2.pdf  3.pdf  joined.pdf

It also could be used for many other PDF manipulations, man for it.