The Optical Character Recognition (OCR) system needs to be set on the Client level, so you'll need to contact your Account Admin to be able to use OCR for your emails. 

If your Client has OCR set, you'll be able to get the search results matching the entered search terms since there might be some attachments containing them.

In case your PDF or image attachments contain the given search term, you will be able to see the snippet in a single email. 

These are the image formats that we support OCR for:

• PNG
• JPEG
• TIFF
• JPEG 2000
• GIF
• WebP
• BMP