A simple Ghostscript command to merge two PDFs in a single file is shown below:
gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=combine.pdf -dBATCH 1.pdf 2.pdf
Install Ghostscript:
Type the command sudo apt-get install ghostscript
to download and install the ghostscript package and all of the packages it depends on.
didn't work for me. silly errors in the output that i did not understand.
but pdftk free did work for me => https://www.pdflabs.com/tools/pdftk-the-pdf-toolkit/
beautifully simple and fast command line. might help someone else
pdftk.exe input_*.pdf output combined.pdf