OK Hi all
I put the question in another way…
how to convert multi paged pdf file in multiple tiff files with desired resolution from command line.
I hope this is more clear 
i search the net… i should do this first so sorry…
imagemagic is a converter among raster images…
convert -density 600 input.pdf output.tiff
is do my job. it takes too many time. and the output.tiff was almost 1 GB size
some people suggesting scripts like pdf>ps>png and they said that this way much faster works but I have no try this…
also I know about PIL library for python and theoretically it should work too…
I am puzzled why you need a density of 600; that is the reason for the very large file.
My scanner usually uses 200 or 300 and the output is perfectly OK for normal use.