PDFUnite is a tool that can be used to join together multiple PDF files into a single output file.
poppler-utils
package which it ships under:$ sudo apt install poppler-utils
in-1.pdf
, in-2.pdf
and so on into a single output file out.pdf
:$ pdfunite in-1.pdf in-2.pdf out.pdf
Tried with: Poppler 0.62.0 and Ubuntu 18.04