There are two main types of PDF documents - those that are created electronically using PDF creation software and those that are created from a scanner or other photo-imaging equipment. PDF creation software actually builds a PDF document that has an internal structure, denoting characters, fonts and position - although the raw information makes little sense to the human eye. A scanned PDF is basically just a flat image of a document - hence, scanning a page of text results in a picture of words being represented on the screen. In order to take information from this sort of scanned PDF, OCR technology is required so that each character can be optically recognized and then represented. Click here to learn more about OCR. You can generally visually determine if a document is a scanned document by enlarging the picture on your screen and looking closely at the text. A scanned image will appear to have much poorer resolution, when looked at closely, than a created PDF document.
This article refers to Able2Doc, Able2Doc Professional, Able2Extract and Able2Extract Professional.