I would definitely be happy about the book and accept it. Even if you don't want to have the book and it would only be on your shelf and regardless of this situation, if someone in life offers you something, they have possibly put a lot of work and effort in making it. I would consider it rude to ask for a digital copy of it, "to save paper" or "because you read it on a PDA" or for any other reason.
Your professor is possibly also a bit older and this does not have to be the case with every older person, but when I tell you that having a physical copy of a book with some ideas from someone, it is a great thing to have it as a book and this will also possibly be the thought pattern of your professor. If the professor wanted to have you a PDF file they would have emailed it to you by now.
If I would be in this situation and I wrote a book and wanted my students to have it and they ask for a PDF file of it I would possibly dismiss the class and hand them a copy of How to Win Friends and Influence People and make them write an essay about it, hehe. Again and again. This not only has to do with usability, but also with respect towards another person's thoughts and feelings.
It is another thing when you ask the professor for a copy. Then surly you can ask to have it as a PDF file or as a book if possible. The other way round this would work, however when being offered a gift in life you go by the simple rule, smile, be genuinely happy about it, gladly accept it and if you don't like it put it on your shelf and forget about it. There is more damage done in being picky about the format of a gift than simply accepting it and not letting the professor know you would prefer their gift in another format.
Sure, one or two weeks after you have accepted the gift you could very kindly ask for a PDF version of the book since you like to "have it with you when you travel on the bus to uni" or something along the line, simple reason, for you to be able to take the information in it with you wherever you are, implying you value the information given to you. This again would possibly flatter the professor and not make them think this student refused my gift and it asking for a digital copy to torrent it, no thanks.
Of course, if your professor is a PDA-loving cool dude/chick that loves the latest tech gadgets, has all their classes in digital format and only reads on PDAs you could surly ask for a PDF file straight away, it depends on the situation and the type of character your professor is.
General advise, not only in this situation but for the rest of your life: If someone offers you a gift, accept it as it is and possibly thank them for it, that depends on your manners.