If you haven't done so already, install wkhtmltopdf as described in Chapter 1, Installing the Odoo Development Environment; otherwise, you won't get shiny PDFs as a result of your efforts.

Also, double-check that the configuration parameter web.base.url (or alternatively, report.url) is a URL that is accessible from your Odoo instance; otherwise, the report will take a long time to generate and the result will look strange.