Click Scan. It's in the bottom-right corner of the window. Your document will begin scanning into your computer. When it finishes, you'll be able to find it in your selected save location.
The default action and often only option of scanning a document is to save it as a picture. To scan the text of a document directly into text that a word processor can edit, you'd need to use OCR (Optical Character Recognition) software.
You can use the Notes app to scan documents and add signatures on your iPhone or iPad.