Using Pdfunite
This method is fast and the resulting combined PDF doesn't loose details. There have been reports though, that the resulting combined file is very big and that clickable URLs in the document are lost.
# pdfunite in1.pdf in2.pdf in3.pdf out.pdf
Using Ghostscript
Alternative 1
# gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=merged.pdf mine1.pdf mine2.pdf
Alternative 2
Should give a smaller merged PDF. Below should be all on one line.
# gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/default -dNOPAUSE -dQUIET -dBATCH -dDetectDuplicateImages -dCompressFonts=true -r150 -sOutputFile=output.pdf input.pdf
Source
https://stackoverflow.com/questions/2507766/merge-convert-multiple-pdf-files-into-one-pdf
