Useful commands:

convert pdfs to jpeg:
for dir in *; 
 do (echo "in $dir"; 
  cd "$dir"; 
  for file in $(basename -s .pdf *.pdf); 
   do echo "converting $file.pdf"; 
   size=$(identify "$file.pdf" | awk '{ print $3 }'); 
   convert "$file.pdf" -depth 16 -size $size "$file.jpg" ; 
  done); 
done