Bulk download and rename file

dl.sh

#!/bin/sh

mkdir RGD_PDF
counter=0
for id in `cut -d "=" -f 2 google_file_links`;
do
    echo $((counter++));
    wget --load-cookies /tmp/cookies.txt "https://docs.google.com/uc?export=download&confirm=$(wget --quiet --save-cookies /tmp/cookies.txt --keep-session-cookies --no-check-certificate 'https://docs.google.com/uc?export=download&id=${id}' -O- | sed -rn 's/.*confirm=([0-9A-Za-z_]+).*/\1\n/p')&id=${id}" -O RGD_PDF/RGD_$counter.pdf && rm -rf /tmp/cookies.txt
done;

create a file 'google_file_links' with all the links to download the file.

bash dl.sh

To generate pdf files with file name 1.pdf 2.pdf..., simply change the 'RGD_$counter.pdf' to '$counter.pdf'.
Then use pdftk to combine all the files in order with the following command:
pdftk *.pdf cat output newfile.pdf