I am assuming you are applying for a TRV ("tourist visa"/"visitor visa")?
For a TRV, a scan of a signed invitation letter should be ok for an electronic visa application. For a paper application, you will probably need a notarized physical version, so you should do an electronic application if you can. At least these were the rules when I last looked at them (a bit more than a year ago), they may have changed. Such rules also tend to vary by visa office (my experience is with several successful visits with visas issued by the Moscow one).
By the way, the other answer says that the invitation letter is "optional". I think this is a bit misleading. On the application form, you have to truthfully state your reason for visiting Canada, and the people you intend to meet in Canada. If your primary reason for visiting is to see friends or family, and you don't have an invitation letter, your application will look very strange, and anything that looks strange and unusual will have a higher chance of rejection. Furthermore, you have to specify how you will afford to live in Canada during your visit, and if you intend to rely on your host (even if it's just for providing you with a place to live), you really do need the invitation letter to prove that the host agrees to accommodate you. Also, while you technically don't need proof of income of the host, it does help to provide it, especially if the visit is long, if you plan on rely on your host, and/or if the relationship is close (e.g. inviting a relative, girlfriend/boyfriend, etc). Finally, the invitation letter is an excellent place to make your case if you word it right; As I understand it, immigration officers read it carefully and it's one of the few places in the application where you have an opportunity to provide free-form information. So unless you're going to visit a complete stranger (e.g. CouchSurfing or AirBnB host), I would strongly suggest including an invitation letter.