Here is another ghostscript trick to split a PDF into separate pages and convert them to high quality PNG files:

gs -dBATCH -dNOPAUSE -dSAFER -sDEVICE=png16m -r300x300 \
   -sOutputFile=output_"%d".png -dTextAlphaBits=4 \
   -dGraphicsAlphaBits=4 ../input.pdf

You can change the output dpi by modifying -r300x300 to dpi of your choosing.