First, in order to start editing the pdf files, you have to install two packages, namely Ghostscript and PDFTK. Now, in order to merge the two files enter the following in the terminal:
gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=firstANDsecond.pdf -dBATCH first.pdf second.pdf
Notice the "AND" between the first and second files.
Now, in order to reduce the size of the pdf files, you will have to type the following in the terminal:
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf
I hope this post is helpful.