https://www.ghostscript.com/download/gsdnld.html
Advanced System Settings -> Environment Variables
System Variables -> New
Variable:GHOSTSCRIPT_HOME Value:C:\Program Files\gs\gs9.54.0
Select Path system Variable
System Variables -> Edit
Add New
%GHOSTSCRIPT_HOME%\bin
Click OK
Added automatically when using sudo apt-get install ghostscript
or Arch
$ gswin64c --version
9.54.0
$ gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=merged.pdf -dBATCH 1.pdf 2.pdf
$ gswin64c -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=combined.pdf -dBATCH 1.pdf 2.pdf
https://www.pdflabs.com/tools/pdftk-the-pdf-toolkit/
$ pdftk.exe *.pdf output combined.pdf