Fujitsu ScanSnap N1800 Operator's Manual page 302

4. Start the scan.
The front and back side images are merged into a double-page spread image.
There may be a gap between the front and back side images, or a line may appear
on the output scan image.
If a thick document is scanned with the carrier sheet, scanned images on the both
right and left sides may be skewed to a trapezoidal shape.
These problems may be improved in the following ways.
Fold the document neatly
Align the edge of the document with the edge of the carrier sheet
Scan the carrier sheet with the front side facing the paper chute or vice versa
The folded part of the document may be mis-detected, causing the seam of the
facing pages be unintentionally trimmed on the scanned image. In this case, the
folded edge of the document should be placed 1 mm away from the edge of the
carrier sheet.
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6.1 Loading Documents