There are a lot of posts about how to combine PDFs using PDFTK, but I had problems when the PDFs were secured. GhostScript (gs) powers right through.
gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE="Combined_PDF_Title.pdf" -dBATCH \ "PDF 1 Title" \ "PDF 2 Title" \ "PDF 3 Title"