The OCR preferences determine the optical character recognition engine used in PDF-Tools:
Figure 1. Preferences Dialog Box, OCR Tab
•Use the dropdown menu to switch between the Default OCR engine and the Enhanced OCR engine. Note that the Enhanced OCR engine is an optional extra in PDF-Tools that was added in version 8, and from version 9 is only available in PDF-Tools via a PDF-XChange PRO license. The Enhanced OCR engine is faster, more accurate and more dynamic than the default OCR engine, and it also contains some extra features. Further information about the Enhanced OCR engine is available here.
•Click Add/Update Languages to add/update languages to be used in the process of optical character recognition.
Click OK to save changes.