gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=output.pdf -dBATCH file1.pdf file2.pdf file3.pdf
Free PDF editing software might become more and more available, but this method has been working for a long time, and likely will continue to do so.
It may not work with all PDFs, for example files that don't conform to Adobe's published PDF specification.