Merge multiple pdfs into one on linux:
poppler package, which comes with multiple tools. The one we are going to use is
pdfunite 1.pdf 2.pdf 3.pdf 4.pdf 5.pdf 6.pdf out.pdf
Note: Do not forget to add the last parameter which is the output file. If you don't give it, it will override the last input file that you give.
pdftk.pdf 2.pdf 3.pdf 4.pdf 5.pdf 6.pdf cat output out.pdf
pdftk pdf1.pdf cat 1-endsouth output rotated_pdf1.pdf