OCR (Optical Character Recognition)
OCR stands for Optical Character Recognition. OCR is used by Acrobat to analyze scanned text and turn it into selectable strings of text that you can copy, as you would any other computer generated document. For the most part you will only see this feature if you are scanning a hard copy into a PDF. OCR typically only works with previously computer generated writing, it can only recognize typed letters and isn't very effective if used with a handwritten document. Another element to consider is that OCR needs to scan the text, and to do that it needs that text to be of a relatively reasonable size. If the font is too small OCR will not be able to determine the separation between letters and will only see a blob of shapes. This can be largely dependent on the quality of your scanner, as higher quality scanners tend to get considerably sharper images that make it easier for OCR.