As you have mentioned above, you are trying to add the watermark to the PDF.
You can find an option to add watermark to the PDF under the "Edit PDF" tool in Adobe Acrobat DC.
Open PDF and select Edit PDF from the right tool pane.
Take help of the steps mentioned here Add watermarks to PDFs in Adobe Acrobat DC
Check if that is what you are looking for.
Let us know if you need any help.
You can right click your toolbar and hit 'Customize Tool Bar' to find the Watermark Tool. It is under the Edit PDF menu.
The button itself is this one
If you save it to the toolbar, it's a drop down button and when you hit it, it will have options to Add, Update, or Remove a watermark.