Perhaps the PDF in which the search is attempted contains no text, and it is a scanned (image-based) pdf. A search will never yield results on an image based PDF, until the OCR process is run to convert from image to text-based content. If you go to Tools>Edit PDF, can you select any of the text? If not, your file is image-based, and a 'find" won't work.
If you find that this is the case, and you would like to try the OCR function, it is in Tools>Enhance Scans>Recognize Text.
Hope this helps. My best,
I have opened the PDF of other machine an am able to search through it. But can't seem to get it to work on this specific machine.