Thank you all for your help and feedback!
The method, after adjusting it accordingly to the picture I have, shows a perfect watermark within Manager.
It took me a bit to figure out that the picture I had was too big but, after resizing it, I was able to make it show as intended.
For the picture that I have and use for watermark these invoices, I had to bring the opacity to 0.09 and play around with the Top and Left margins in order to get it centered.
Regarding the PDF permissions, the only PDF tool I have is Adobe Acrobat DC (15.23.20056), the latest and updated version of Acrobat Reader for Windows 10.
And I do work fine with it. (or so I think!)
I have tried to save word documents with watermarks into PDF and it worked well.
Just tested it and the same issue about the watermark being placed on another page without any watermark properties, happens also when emailing the file within Manager, at least to me.
I have tried with the Notes box and also with an unnamed custom box so that it looks a bit cleaner.
Again, and as I have mentioned before, I recognize that this might be a case of me not doing something right but, for now, I will have to live with it due to lack of time to try to figure this out.
So… for now, and until the solution for the issue, I am just sending the Invoices without the Watermark.