To create image thumbnails from a PDF document, run this in a terminal window:

convert -thumbnail x300 -background white -alpha remove input_file.pdf[0] output_thumbnail.png

The parameters to convert do the following things:

Parameter Effect
-thumbnail Similar to -resize, but optimized for speed and strips metadata.
x300 Make the thumbnail 300px tall, and whatever width maintains the aspect ratio.
-background white Sets the thumbnail background to white.
-alpha remove Removes the alpha channel from the thumbnail output.
input_file.pdf ...

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