Brother MFC-3420C - Manual (Page 39)

2 - 2 LOADING ORIGINALS AND PAPER
2
Fan the pages well and stagger them at an angle. Make sure
you put the originals face up, top edge first in the ADF until you
feel them touch the feed roller.
3
Adjust the paper guides to fit the width of your originals.
DO NOT pull on the original while it is feeding.
To use the ADF, the scanner glass must be empty.
Approx. 1/2 inch