How to merge PDF files using PDFTK

PDFTK can be used to easily merge or stitch or combine together multiple PDF files into a single PDF file.

To install PDFTK, please follow the instructions here. To merge PDF files, commands like this can be used:

$ pdftk *.pdf cat output out.pdf

$ pdftk 1.pdf 2.pdf 3.pdf cat output out.pdf

Tried with: PDFTK 1.41