You can add a Download Button by using an add-on/extension/plug-in available in the add-ons gallery available for every browser.
For Chrome you can use the WebMarket filled with extensions, for Firefox you can use the Add-ons gallery and for the Internet Explorer browser you can use Iegallery's add-ons. Back in the days Opera used widgets but now they use extensions, just like Chrome and the website is accessible by following this link.
These are the most common and most used browsers that you can use with support for add-ons.