Ghostscript can be used to merge PDF files into one document.

Here the instructions for macOS: Install Ghostscript with Homebrew and run it afterwards.

$ brew install gs
gs -q -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=merged.pdf *.pdf

Source: https://personalinterests.lipingyang.org/2018/01/12/two-ways-to-merge-pdf-files-on-mac-gui-and-command-line/