Joining PDFs into a single file

One of the easiest ways to join a number of PDFs into a single file is to use the pdftk command.

A pdftk command to join a number of PDFs into a single file might look like this:

$ pdftk recipe-1.pdf recipe-2.pdf recipe-3.pdf recipe-4.pdf recipe-5.pdf cat output recipes.pdf

You can use a wildcard if your files follow a regular pattern like this:

$ pdftk recipe-*.pdf cat output recipes.pdf 

https://www.networkworld.com/article/3609508/creating-and-merging-pdfs-on-linux.html 

 workaround

sudo ln -s /snap/pdftk/current/usr/bin/pdftk /usr/bin/pdftk