Combining pdf files in linux

One of the tool I always use to combine pdf file, is pdftk.

To install pdftk, just run apt install

$ sudo apt update && sudo apt install pdftk -y

To combine pdf files into one:

$ pdftk file1.pdf file2.pdf file3.pdf cat output combined.pdf

where file1.pdf , file2.pdf and file3.pdf are the outputs, and the combined.pdf is the result after the combination process.