Add Watermarks to PDF Securely Online

Adding a watermark to a PDF involves injecting an overlay content stream into each target Page object without corrupting existing text or graphic operators. Khalasna's client-side engine parses the PDF's internal resources dictionary and injects a new graphic state parameter dictionary (/ExtGState) defining alpha blend opacity (/CA and /ca values). For text watermarks, it constructs a vector text showing stream positioned via affine transformation matrices (cm operator) to handle scaling and rotation (typically 45 degrees). For image watermarks, it registers the logo as a sub-XObject Image and paints it using the Do operator. By appending these commands to the end of the page's /Contents array, the watermark is rendered cleanly above the document background while preserving selectable underlying text. All binary modifications occur locally in browser memory using WebAssembly, ensuring strict document privacy.