$ pdfunite ES_0818_1.pdf ES_0818_2.pdf ES_0818_3.pdf ex_082018.pdf
In order to remove an (unknown) password from a protected pdf use this command:
$ gs -q -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=unencrypted.pdf -c .setpdfwrite -f encrypted.pdf