I have been asked to develop a new feature for Sharepoint Server 2013 on premise which I have no idea how to implement. It basically consists on "catch" any user attempt to download/open a PDF file on a specific Sharepoint site, add a watermark behind the scene, and deliver the newly edited file to the end user.
I have edited PDF documents programmatically hundreds of times using C# libraries, but I don't know how can I manage to override the default behavior to achieve this requirement.
Can anyone please give me some guidance? Thanks in advance.