The most elegant, open-source and completely free solution: use ghostscript.
$ gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=finished.pdf inputfile1.pdf inputfile2.pdf
ghostscript (gs) is pre-installed on most Linux distributions.
Learn about Merging multiple pdf files and jpg files into one PDF in this related post where files of different types (JPGs and PDFs) are merged into one PDF.
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