Try the good ghostscript:
gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=merged.pdf mine1.pdf mine2.pdf
or even this way for an improved version for low resolution PDFs (thanks to Adriano for pointing this out):
gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -dPDFSETTINGS=/prepress -sOutputFile=merged.pdf mine1.pdf mine2.pdf
In both cases the ouput resolution is much higher and better than this way using convert:
convert -density 300x300 -quality 100 mine1.pdf mine2.pdf merged.pdf
In this way you wouldn’t need to install anything else, just work with what you already have installed in your system (at least both come by default in my rhel).
Source: Stack Overflow