PDFTK
Example
# concat specific pages
pdftk A=doc1.pdf B=doc2.pdf cat A1-4 A5 B1-end A6 output out.pdf
# concat all pages
pdftk doc1.pdf doc2.pdf cat output out.pdf
Remove password from pdf file
pdftk input.pdf output output.pdf user_pw PWD
# concat specific pages
pdftk A=doc1.pdf B=doc2.pdf cat A1-4 A5 B1-end A6 output out.pdf
# concat all pages
pdftk doc1.pdf doc2.pdf cat output out.pdf
pdftk input.pdf output output.pdf user_pw PWD