There are usually two steps necessary to look at invoices (or other PDF documents): first the download of the document and then the opening in the PDF viewer. For those who actually just want to have a look at the document (without printing) there is an easier way.
Google Chrome has a plugin that allows you to display the PDF file directly in your browser window.
After the installation of this plugin, you can decide if you just want to see the PDF document or if you want to download it.