If you don't have control over the site you are linking directly, then direct linking would be a bad idea. Because whenever the external site goes down, you can't retrieve the e-book or refer your visitors to that PDF file. i.e. You are at the mercy of the external site. If the e-book is important for you, keep a copy in your local hard disk and upload it in a server that you have control of, and then link to the e-book in your own server. Sharemation.com has long been my favorite for uploading and sharing files. However, you get only 5MB. Esnipes.com gives 1GB space, you can try them also.