How big are the pages?  How big is the type?  Do you just wantt ot read what is on the
page, or do OCR.

The OCR program I use wants pages to be scanned at 300 ppi.  For an 8X10" page, that is 7
Mp.  To just read the type, 150 ppi is probably enough, or 2 Mp.

If all you want to do is read the document on the screen, 3 MP is a big
plenty.
For viewing on the computer only, it is maybe too much.

If you want make a editable document on the computer, it is not enough.
OCR programs need 300 DPI to convert the text.

For a 8.5 X 11 inch document that is 2550 x 3300 Pixels. Which is 8.415
MegaPixels.

A cheap flatbed scanner is best for A4 or Letter Size documents.