The OCR functionality is switched on/off in the Options tab of the Expert search. Make sure you have switched on OCR Images:
Then switch to back to Index Search and open the Index Manager. You'll need to re-create any indexes that you want to use with the new OCR settings.
PDFs with text
The default OCR functionality is to ignore PDFs that already have text. The assumption is that if the PDF already has text the file does not require OCR. You can change that behaviour in the OCR Settings by unchecking the option Do not OCR PDFs with existing searchable text: