For Odoo versions 8, 9, and 10 there is a very good free module which allows printed reports such as Invoices to be previewed in a new browser window rather than the PDF being downloaded each time Print is pressed.
\max_web_pdf_preview
There have been so many changes to how Reports are done between V10 and V11 that it is beyond my programming ability to make it work in V11. Has anyone developed, or is anyone aware of an equivalent module for V11?