I.R.I.S. Readiris 14 for Mac User Guide - Manual (Page 69)

Section 6: Optimizing the Scanned Documents
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If the image was made from an angle, you can correct its
perspective with the Deskew icon.
Notice that the Deskew icon looks different for digital camera
images.
To deskew an image:
 Select the image, then click the Deskew icon.
 Drag the corners one by one in the direction of your text.
Make sure the top and bottom sides are parallel to the text.
Make sure the right and left sides are parallel to the text margins.
 Click Apply to deskew the image.