When a user clicks on a diagonal (45°) watermark in Adobe reader or Acrobat, the watermark may become invisible. It will reappear when the user changes pages, but will become invisible again should they click the same diagonal watermark on subsequent pages.
This issue occurs because PDF "form fields" can only be rectangles and have to be parallel to the page. A watermark "form field" is required when a "User Specific" watermark is defined AND the placement is diagonal or 45°. A "User Specific" watermark is one where field variable placeholders are referenced as opposed to only static text.
We recommend using vertical or horizontal "User Specific" placed watermarks instead of diagonal watermarks if you are distributing downloaded PDFs to your customers.