Not knowing how you created these files, it is difficult to know why X1 cannot highlight text in the viewer pane. I have never encountered that with "traditional" PDF files. It always highlights search terms.
These are the files that don't consistently highlight for me in anything other than a PDF reader, any reader. (I don't have Adobe Reader on my PC.) Strangely, though, it sometimes flags up the search term when I use the Next match button at the bottom of the viewer pane.
You might be able to get the text to highlight if you did away with the image overlay. In other words if you used the OCR process to create a text-only PDF. But then you lose the look of the original document.