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Sometimes somebody send you a pdf with big white margins that only annoy you. This is often when you receive Power Point presentations in pdf format so is almost impossible to print several slides on one page.
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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:
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:
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.