Combining multiple ps pages into a multiple page pdf file

Problem
You have a lot of ps files and you want to combine them into a single pdf file with multiple pages.

Solution
You can use Ghostscript:

gs -q -sPAPERSIZE=a4 -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=out.pdf in1.ps in2.ps in3.ps