Combine multiple PDF files

I sometimes want to combine two or three PDF files into one (to print them two pages on a sheet, mostly).

I did this recently and here I write it down again so I can find it again (I will forget this until the next time I need it again):

$ gs -q -sPAPERSIZE=letter -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=output.pdf file1.pdf file2.pdf file3.pdf ...

Found on Stack Overflow.

I also found another solution (further down at the link above) usable, but it produces much bigger files:

$ qpdf --empty --pages *.pdf -- out.pdf