How can you scan a book and save as a pdf file that won take a lot of space on your hard drive?
The problem is you are scanning an image of the pages… so you’ve got a pdf that describes a picture of the page not the words on it. You can maje it smaller by scanning at lower resolution… less dots per inch means less dots per page image. You can make it even smaller by scanning to a word processor (going through OCR) and then making a pdf out of the document. That way you’ve got a pdf that describes the places that letters are on the page and not a picture of the letters. But that’s a long process and you’ll need to check all of the text to correct any errors in the OCR process. Oh, and you are in breach of the copyright rules of the book if you copy a substantial piece of it.
