I.R.I.S. Readiris 14 for Mac User Guide - Manual (Page 135)

How To
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 On the Readiris menu, click Preferences.
 Click the General tab.
 Select the option Process as 300 dpi.
When this option is selected, Readiris treats the images as if they
had a 300 dpi resolution.
Tip 2 (Readiris Corporate only)
Make sure that favor OCR accuracy over speed when processing
low-quality documents:
 Click the language list on the top toolbar and select Other
languages.
 Make sure the option Favour recognition accuracy over speed is
selected.
Note:
When you are processing low-resolution faxes, there is no need to
take special measures. Readiris detects the typical fax resolutions
100 x 200 dpi (normal quality), 200 x 200 dpi (fine quality) and 200
x 400 dpi (superfine quality) and preprocesses these images
automatically to ensure optimal results.