Scan, as in an actual scanner to scan a paper copy into a computer file?
If that's what you are doing, then you'll first need to use an Optical Character Recognition (OCR) application to detect the letters/numbers and create a text document. Then you will probably have to edit that document because the software won't detect everything perfectly. Then you have to conver to PDF so that the text can be searched.
If you don't convert to a text file from the scanned image, the PDF will only have the image in it, and the image can't be searched like text.
Personally, I've never been impressed by the OCR software I've used. Good luck to you though.
