Merge multiple pdfs into one on linux:
Use poppler-utils or poppler package, which comes with multiple tools. The one we are going to use is pdfunite.
Example:
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.
Using pdftk:
pdftk.pdf 2.pdf 3.pdf 4.pdf 5.pdf 6.pdf cat output out.pdf
Rotate 180:
pdftk pdf1.pdf cat 1-endsouth output rotated_pdf1.pdf
Source: here