Canon 2921B002 On-Screen Manual page 388

Scanning Images Larger than the Platen (Stitch Assist)
Select Stitch Assist for Document Size, then specify Scanning Resolution as required.
5.
Return to the Scan/Import window and click Scan.
The Stitch-assist window opens.
6.
Place the left half of the document face-down on the Platen.
7.
Click Scan.
The left half of the document is scanned and appears in the Stitch-assist window.
8.
Place the right half of the document face-down on the Platen.
9.
Click Scan.
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