Currently, the watermarking feature only works with image files (e.g. JPG, PNG) and does not support PDF files. This is a significant limitation, especially considering how widely the PDF format is used in our digital collections.

https://www.youtube.com/watch?v=_xqzt4eZcb4

We strongly encourage you to remove this restriction and implement support for watermarking PDFs: it’s an important improvement for our workflows.
Thanks