Thanks for the response, butI have to rephrase my question: Is there a better way to get a better quality/image size (in bytes) ratio?
One example: I've converted the same scan with two different options to approximately the same size (380 kB):
1st Conversion:
convert -resize '1680x1680>' page26.pnm page26-1680x1680.jpg (uses standard compression 85):
2nd Conversion:
convert -quality 27 page26.pnm page26-q27.jpg (this sets the "JPEG compression" to 27 without changing the image size):
Which one is better?
Of course proposals for other programs which produce better results are welcome