Canon MX870 Series User Manual page 506

Click Specify....
5.
Check that the Automatic Duplex Scanning checkbox is selected, then specify how to output the
paper, the orientation of the documents, etc. as required.
When setting is completed, click OK.
Scan Settings Dialog Box (Stack of Documents (ADF))
Click Scan.
6.
The back side of each document is scanned automatically after scanning the front side.
Note
Click Cancel to cancel the scan.
When scanning is completed, the Scan Complete dialog box opens. Select Scan or Exit. Select
Scan to scan the next document, or select Exit to end.
The scanned images appear in the Thumbnail window.