I'm not able to reproduce any issues with The World's Simplest Calculator. Let me try to reproduce the steps to creating a report: You go here:
After clicking "create report," you get the following result:
From here you click "Download PDF Report" and it will download the PDF to your default file download location, or open it in your default PDF viewer depending on your system settings. From there you can move it and treat it like any other PDF file.
Let me know if I am missing a step, or if there is another stage at which you are experiencing the issue.