PDF merging utility based on pdfjam. More information: https://github.com/rrthomas/pdfjam-extras.
pdfjoin path/to/file1.pdf path/to/file2.pdf
pdfjoin path/to/file1.pdf path/to/file2.pdf ... 1 --outfile output_file
pdfjoin path/to/file.pdf 3-5,1 --suffix rearranged
pdfjoin /path/to/file1.pdf 2- file2 last-3 --outfile output_file