This guide shows instruction about joining some PDF files into one with pdfunite utility. It is built-in in Ubuntu along with poppler-utils package. Thanks to Poppler Project and Glyph & Cog.
pdfunite <pdf_file_1> <pdf_file_2> <pdf_file_n> <final_pdf_name>
Explanation: pdfunite will treat any file name you enter as argument, as input. And the last file name you enter, as output. pdfunite will arrange files according to your arrangement.
pdfunite packagingguide.pdf ubuntu-desktop-guide.pdf xubuntu-desktop-guide.pdf result.pdf
Explanation: this command line will join three PDF files into result.pdf file.