I have encountered problems using the 'search' function in Adobe 9 Reader. Some pdf documents I've received allow me to use the 'search' function, but others do not - Adobe Reader flags up that the word/s can't be found, although they are in the document. I've looked at Adobe Writer but I can't see anything that would allow the writer to disable the Search function when creating a pdf document. ??
Without OCR there is nothing for Find/Search to "see".
With OCR, well OCR can often be less than 100% and some characters/words will not be present in the OCR layer of hidden text.
Or the OCR character (a "best-estimate") may not match what is in the image.
Be well...
Be well...