Include watermark in secure view mode

[ediazcomellas]

Secure view of PDF files is a great feature to have. Although avoiding completely the copy of contents may be a daunting task, there are some useful steps that can help to make "secure view" more robust. One of them is to watermark the documents being displayed, so even if the user makes screenshots of the content they will have to work a lot to remove the watermarks.

Collabora is already allowing this, but they are more oriented to office-like documents, not just PDF viewing. It would be great if PDF viewer could implement this watermarking feature for secure views.

[szaimen]

This might work: https://github.com/mozilla/pdf.js/issues/7783#issuecomment-346534883

[carlos837]

Please implement this

[naifqarni]

much needed function