Imagemagick's convert

convert -density 300 -antialias a.pdf -background white -alpha remove a.png

Ghostscript

This will create transparent images; I haven't found out how to remove the alpha and add a white background.

gs -q -dQUIET -dSAFER -dBATCH -dNOPAUSE -dNOPROMPT -dMaxBitmap=500000000 -dAlignToPixels=0 -dGridFitTT=2 -sDEVICE=pngalpha -dTextAlphaBits=4 -dGraphicsAlphaBits=4 -r300 -sOutputFile=a-%d.png a.pdf

Bonus (back to PDF)

convert a*.png b.pdf