I.R.I.S. Readiris 14 for Windows User Guide - Manual (Page 74)

Section 7: Adjusting the Image Quality
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To improve the recognition results:

Check the Scanner Settings and make sure your documents
have been scanned in a resolution of 300 dpi and in grayscale.
See the section Selecting the Scanner Settings if necessary.

If that's not the case, re-scan your documents using these
settings.
Then access the Image Adjustment options:

Click the View tab.

Clear the option Display Document in Color.

Then click Size > 100 %.
This enlarges the image, allowing you to inspect the text
characters. The characters should not be touching each other,
filled in, faded, or broken.

Now click the Page tab, and click Adjust to start adjusting the
image.