Making use of a PDF editor tool that offers PDF watermark feature will be best fit for this purpose. In my opinion and as far as my experience is concerned, I would suggest you to make use of Wondershare PDFelement for adding or removing all kinds of watermarks to/from PDF on Mac computer. Wondershare PDFelement is a multi-functional PDF editor which enables you to quickly create, edit, share, read, convert, sign and annotate PDF documents. So, let me share a short video tutorial with you that will help you to learn how to add or remove PDF watermark on Mac using PDFelement. You need to simply follow the steps as explained in the video to add text and/or image watermark(s) to one or multiple pages of a PDF document on Mac computer. In case of any issue, you can leave a comment below the video.