Alfresco does not convert images using OCR to allow searching of text. You would need to do this yourself, then uplaod (maybe searchable PDF) or look to integrate an OCR engine. There is (was) a company that had integrated and OCR engine, http://www.intelliant.fr. You could check that site and see if they have kept it up to date.
We have implemented an OCR server integrated with Alfresco, which can be used as transformer or via Javascript and Java. It runs on a separate OCR server and supports Abbyy and Google OCR. for more informaiton see here - https://forums.alfresco.com/en/viewtopic.php?f=33&t=44739