I.R.I.S. Readiris 14 for Mac User Guide - Manual (Page 134)

Readiris TM 14 – User Guide
129
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RESOLUTION IMAGES
A resolution of 300 to 400 dpi is required to obtain optimal results
in Readiris. When you try to scan or open images of a resolution
lower than 200 dpi or higher than 600 dpi, Readiris will display a
warning.
When you are faced with these kind of images, the best thing you
can do is re-can the images in an appropriate resolution. If that's not
possible, there are few tips and tricks that help you obtain better
results:
Tip 1
When scanning images of an unknown resolution or images of
which the file header displays an incorrect resolution, make sure
to select the option Process as 300 dpi: