利用 imagemagick 即可

但是轉出來的效果就不怎麼樣, 經過下面的解釋: http://www.imagemagick.org/Usage/text/#postscript
As you can see postscript converted to the default "-density" of of 72 dpi, often does not look as good as it should, with only a minimal amount of anti-aliasing. This is particularly the case when dealing with postscript fonts, which are not designed to work at these low-resolutions. To improve this, you can have "ghostscript" draw at a higher resolution (or image "-density"). You can then "-resample" (a specalized resize using image densities) this larger image back to a more 'normal' screen resolution density. The value '196' is 3 times the final 72dpi, which means when "-resample" is used approximately 3×3 pixels are merged down into each pixel. This produces better anti-aliasing pixels along the edges of the text improving the overall look of the result.

所以我需要 pdf viewer 裏面 300%  顯示的高解析度圖形的話,
就這樣使用:
convert -density 196 a.pdf a.jpg

太棒了!