MacOS Combine PDF's Documents

Combining multiple PDF documents on MacOS is included with MacOS.

Turns out Apple included an Automator script for "Combine PDF Pages" with the OS and a Python script in the same folder.

So from terminal, you simply call the Python script and supply the output file (-o) with the input files.

/System/Library/Automator/Combine\ PDF\ Pages.action/Contents/Resources/join.py -o {output.pdf} {input1.pdf} {input2.pdf} {input3.pdf} {...}

A lot simpler and quicker than some other solutions.