PDFs come in many different formats. Some require more complex processing to get the text out of them. So you can highlight and search them.
OCR refers to the conversion of scanned or image PDFs into text. Lateral has OCR built in! Depending on your subscription you get different amounts of OCR credits that you can use.
If the highlighting on a document looks off or Lateral shows a warning (purple triangle) on a document, you can simply use the Use OCR button in the Document View.
The document will be processed again and in most cases everything should now work as expected. :)