Arguable, one of the top pet peeves of any PDF user is the inability to search PDF text in overly long PDF files. This is especially true when it comes to handling a scanned or image PDF document. This is because scanned PDFs do not contain letters and words that are recognized as digital text. They are only an image of text and letters with no real data behind it.
For this reason, there is a special way to make a scanned PDF document searchable. This begins by making the text recognizable by using OCR (Optical Character Recognition) technology. Once this is done, you can search PDFs properly. With Acrobat 11 Pro, you can make text recognizable in a single document or in a number of documents.
You can modify the settings by clicking on Edit.