Text, Images and Images of Text
If the PDF contains images it will extract them. If it contains text it will extract text. This sounds simple, but when a text document is scanned, text is becomes images. A scanner actually takes a high quality photo of the document.
To turn images into text you need Optical Character Recognition software, which will take a while to learn to use efficiently.
Is It Scanned?This is easy to check. Drop the PDF into Preview and zoom in to see the detail of letters.