Add the following to your ~/.bash_profile for simple merging of PDFs:

pdfmerge() { 
    gs -dPDFSETTINGS=/prepress -dBATCH -dNOPAUSE -sDEVICE=pdfwrite -sOutputFile=$@
}

Usage:

pdfmerge merged.pdf first.pdf second.pdf

Source:
https://stackoverflow.com/questions/2507766/merge-convert-multiple-pdf-files-into-one-pdf