The PDF, by its nature, is a read-only format. Yeah, you got the E Sign Pdf that allows you to modify it, etc.; and the Reader allows for some markup, but it was initially designed to be a global standard for read-only documents and/or digital forms that you can fill out. As such, people don’t create documents in PDF — t create them in some other app (MS Word, Apple Pages, etc.) and then export them to PDF to be readable on any device. So what you need to do is add the watermark to your document in your designer app, and then export it to PDF together with the watermark. If you don’t have access to the source document, only the PDF — then chances are you are not the creator of that PDF, so you don’t really have any right to add a watermark to it. The above holds true for Mac, Windows, Linux, etc.
PS : If anyone could help me on this, I'm going to make an app with this .pdf file: I will then send it to you personally to edit it on the iPad or your PC. I will also make a free mobile PDF reader app that will let you convert any type of PDF into PDF document, and you just need to drag-and-drop it onto the app to open it. This will make it easy to read any type of PDF.