Some time ago someone asked how I could produce good quality scans of service-manuals but yet keep the PDF files fairly compact.  So, here's my method:
1) My scanner is a basic cheap flatbed scanner (Canoscan 3000/USB). I use the Canoscan Toolbox 4.1 with advanced settings enabled. Your scanner probably has similar functionality as my Canon does, so you will just need to find how to make the settings as I have referred to below.
2) I set "Save as type" to PDF. The scanner will scan to BMP, which in turn will be combined into a single PDF-files by the software when scanning is complete. The BMP’s are deleted and the PDF is your end-product.
3) I always do a Preview to see if the page is aligned correctly.
4) Most text and drawings can be scanned in Black & White / 200 - 300dpi. You should experiment with the Half-tone settings. Default for mine is 72 but scans are better with a setting of 125. It produces a higher contrast, and filters out binder-holes and wrinkles, etc.
5) Schematic Diagrams should be scanned in 400 - 600dpi Black & White, so they can be enlarged by the user without loosing information or getting jagged. If scanned in color, you scan at no more than 300dpi to avoid excessive file sizes. Gray-scale scanning is great for reproducing photos of the interior or for reproducing PCB’s.
The below table gives and indication of the relationship between color setting, dpi setting, and resulting file size:
Color at 300dpi: 24Mb/page
Black and White at 300dpi: 1Mb/page
Grey-scale at 100-200dpi: 1Mb /page
In short, keep scans black and white unless there is detail that requires gray scale or color to display. Also, make sure schematics are done at high dpi's!
Do some experimenting to acquire some expertise before going "final".
6) Before finishing the scans, be sure to make a back-up copy of your BMP’s for safe-keeping. The Canon toolbox will delete every one of them upon exit.
7) You now have your very compact, good quality high-res PDF-file ready for sharing.
In case your scanner software doesn’t allow you to make PDF's directly as described, you should:
8) Use Windows-XP Picture & Fax Viewer to display the first BMP-file (you need to know the save location of your scanner software).
9) Click the Print-icon & mark all files.
10) Select printer -> PDF995, a .pdf-driver that is available here:
Use special print settings for the driver; click on the button. Now select Black & White and Advanced. Set Print quality at 144dpi in the drop-down box. Click OK and return to print everything, giving it a name finally.
11) The result is not as compact as what the scanner toolbox will produce, but still tolerable.
Comments are most welcome...
Kind regards,