Warning, start of geek

mode
100dpi will generally do, Greyscale also can cut down a lot especially if you can reduce the palette size (less shades of grey).
GIMP is a well known graphics program which works on a variety of operating systems.
My own strategy for scanning is to record everything at 300dpi with 24 bit colour and save the result as a PNG file. A set of such scans from a book or manuscript form a master copy for archival purposes. I don't edit these files at all except to rotate them by +/- 90 degrees as appropriate.
If the page does not contain photographs, my recipe for a more 'economical' copy is as follows.
I crop the image to get rid of edge effects,
convert to 100 dpi
convert to grey scale (desaturate) based on luminosity.
Use the colour editor to make the very light greys, white and the darkest greys slightly darker, but not completely black.
I then 'posterize' to reduce the number of shades of grey to 16 or 8
change the image to ' palette' mode
save as a PNG file with maximum compression. (this is a lossless compression.)
this will take a 4 - 6 MB image down to 60 - 100 KB, which I think is far more manageable.
If anyone is still reading this I would welcome any comments or questions.
