There are 4 options to get the report result:

1)      Showing the result in browser viewer.

2)      Direct download as PDF document

3)      Direct download as Excel  document

4)      Direct download as Word document

This can be setup per report:

Go to admin report page and select your report.
On edit page you can set your output format.
When this option is blank the rdl report will be shown in the browser viewer.