First find all the files you want to convert and store their filenames in a file

find my-pdf-folder/ -type f -name "*.pdf" > pdffiles.txt

Iterate through that files and make a jpeg from them

while read -r LINE; do pdftoppm -f 1 -singlefile -jpeg "$LINE" "$LINE"; done < pdffiles.txt

Finally move all jpgs to a separate location if necessary

find my-pdf-folder/ -type f -name "*.jpg" -exec mv {} jpgs/ \;