insert blank pages in a pdf

combiner.sh

i=0
pdftk A=mailmerged.pdf cat A1-3 output combined.pdf
while [ $i -le 116 ]
do
        p1=$(($i*3+1));
        p2=$(($i*3+3));
        echo "$p1 to $p2"
        #pdftk A=combined.pdf B=blank.pdf C=mailmerged.pdf cat A B1-1 C$p1-$p2 output combined.pdf
       	echo "A$p1-$p2 B1-1" >> com.txt
        
	((i++))
done
pdftk A=mailmerged.pdf B=blank.pdf cat `cat com.txt` output combined.pdf