Most scanners come with OCR (Optical Charater Recognition) software like Omni Page Limited Edition etc. When you scan a document is recognises the letters as text and not the whole page as a photo (best way to explain it). Usually when setting up the program it links itself to your Word Processing program, MS Word perhaps. Then when you scan your document page by page Omni Page opens MS Word and the document is ready for editing. Depending on the program, the state of the orginal etc you may have to do some editing.
Even if you scan all pages to separate documents you will be able to produce one document using your Word Processing Package's cut and paste commands.
There is a free OCR package, unfortunately I've forgotten its name, but with that one you have to open the OCR package, open your Word Processing Package etc, it looked a bit more long winded and if it works it's free.