In order to merge several pdf files under GNU/Linux, I found a quick and reliable solution, with the pdftk tool.
The process is very simple:
# apt-get install pdftk
Then let’s say we have 2 pdf files called file1.pdf and file2.pdf
With the command line, just write
~ pdftk file1.pdf file2.pdf cat output file-output.pdf
And you are done!