Post date: Feb 04, 2016 8:5:53 AM
gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=merged.pdf 1.pdf 2.pdf 3.pdf 4.pdf 5.pdf 6.pdf
Above script is a ghost script. usually available under cygwin and linux distros as package.
Alternatively latex include pdf can also do same job. however a latext package for example TexWorks must be installed in system.