To convert PDF to big images like maps you can try with Gimp but it eat too much memory and is a bit slow.
The other option is to use ImageMagick:

felipe@funstation:$ convert file.pdf file.jpg

If the output image is to small you can try to change the pixel density (dpi) which is 72dpi by default to a bigger one.
Here is an example using 500dpi, 8 bits of color (256), inverting the colors (negate) and saving it in PNG format:

felipe@funstation:$ convert -density 500 -negate -depth 8 base_actualizado_diciembre_2009.pdf base_actualizado_diciembre_2009-500-invert-8bits.png

The sample file is here: http://www.osornochile.cl/webimo/documentos/base_actualizado_diciembre_2009.pdf
The PDF size is 7.5M and the output PNG size is 59M.