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