The PDF Report Generator gives instructions that under the Settings, you can choose PDF Destination. Even though you may pick “Browser” as the destination, the module still creates a temporary pdf file in _lib/tmp folder.
These files are not automatically removed and can build up.
The security issues comes if you do not know this and the PDFs are of potentially sensitive information. For a web application that is built with this system and deployed online, the files can now be directly accessed via the web by going directly to the folder.
Therefore, my suggestion to the development team is if “Browser” is selected, that would means that the PDF is dynamically generated and view able only in the browser (No temp PDFs).
As an aside: I have created my own custom PDF modules by creating a new Blank Application and accessing fPDF directly. In this way I have outputed the PDF file only to the browser without any files being saved in my system.