To combine multiple pdf files in a given order:
gswin64.exe -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=Merged.pdf -dBATCH 1.pdf 2.pdf 3.pdf
To import pdf file names from a text file (space separated file names):
gswin64.exe -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=Merged.pdf -dBATCH @filelist.txt
