Sometimes you have a downloadable content on your website that you want your readers to download to their PC, and you don't want the content to be opened in their browser, a pdf particularly. One way to avoid that is to convert your file to a zipped format.

So that' it, you now have a downloadable digital content on your sidebar!

UPDATE: 30th November 2015, I have received some feedback regarding this, some of you had asked if you can add a picture with downloadable content on your page or post. Well you can, while you are editing, go to the "Text Editor" (Not the Visual Editor), and paste the similar coding, the code is now embedded into your post, and you can continue writing by going back to Visual Editor.