ls -1 *\.pdf | while read old_filename
do
        part1=`echo "${old_filename}"|awk '{print $1}'`
        part2=`echo "${old_filename}"|awk '{print $2}'`
        new_filename="${part1}_${part2}.pdf"
        # Remove echo when tested
        if [ ! -f "${new_filename}" ]
        then
                if [ -f "${old_filename}" ]
                then
                       echo mv "${old_filename}" "${new_filename}"
                fi
        else
                echo "Error: Duplicate new filename: ${old_filename}"
        fi
done

ls * | while read a
do
b=$(echo "$a" | sed 's|^\([^ ][^ ]*\) \([^ ][^ ]*\) .*|\1_\2\.pdf|')
eval echo 'mv "$a" "$b"' >rename.sh
done