From Electron Cloud
To render one page of a PDF to PPM on stdout (for piping into another program, for example):
gs -q -dFirstPage=5 -dLastPage=5 -dTextAlphaBits=4 -dGraphicsAlphaBits=4 -sDEVICE=ppmraw -r90x90 \ -sOutputFile=- -sNOPAUSE file.pdf -c quit > file.ppm