The size of a scanned document depends on what the scanner it set to. Scan mode & DPI affect the file sizes Some scan modes: color, gray-scale and black and white. 16-bit Color, 300 DPI will look great but create huge JPG and PDF files. Black and white at 100 DPI will be grainy but create very small files.
Some of our clients use black and white at 300 dpi and found that these settings are good enough to produced a re-printable certificates that are small enough to e-mail. Files created with these settings average .04MB per PDF page. Our system uses JPG files to allow for a quick view of the scanned document. The JPGs average 50-150KB per page
When a client needs or prefers higher resolution or gray-scale the file sizes increase. Some prefer Grayscale at 150dpi, these settings are good for reproducing the documents. At this size you run the risk of having problems e-mailing a multi-page PDF of a certificate. The average sizes are 2MB per page for the PDF and 100KB per page for the JPG.