How to watermark many PDF files at the same time

I needed to add a watermark to 15 PDFs’ at the same time and didn’t want to go through each individual file and merge it with the watermark.

Additionally, if you do not have an Adobe Creative Cloud license it becomes a bit of a challenge to modify PDF files (As Adobe Acrobat DC locks down features for free users).

This is my watermark file, in yours you can be more subtle and change the color to light gray as well as decrease the opacity