The Add Watermark feature allows users to add custom watermarks to PDF documents and set watermark text, font size, rotation angle, opacity, and position. This is useful for users who need to mark or certify PDF documents. For example, in legal documents, users may need to add "Confidential" or "Reviewed" watermarks; in contract documents, users may need to add company logos or signature watermarks. Additionally, this feature can be used to add timestamps or version numbers to PDF documents to track document modifications. By using the add watermark feature, users can ensure document integrity and credibility and easily manage and distribute them.