Report file is an empty file that will contain your report once you have populated it with visual and non-visual elements. Report files have an extension - .rptdesign - and a complex structure described here.
BIRT report files (as 4gl programs) cannot exist beyond a project. You can add your reports (= report files) to the already existing projects (both 4gl and non-4gl ones)or create a separate report project to contain your reports.
Refer here to read about creating new projects.
Refer here to read about creating new report projects.
To create a new report file, please follow these steps.
This video offers a short overview of how to create a report file: