PDF tools
Add Watermark to PDF
Stamp a text or image watermark with adjustable opacity, rotation, size, colour and tiling.
Runs locally in your browser
pdf-lib (MIT) rewrites the documents and PDF.js (Apache-2.0) renders and extracts them; both load only when you start working. Files never leave the browser. Icons plus runtime cmaps and standard fonts come from jsDelivr. Chinese page numbers and watermarks are drawn on a canvas and embedded as bitmaps because embedding a CJK font would mean a multi-megabyte download — so those stamps are not selectable and soften at extreme zoom, while Latin text and digits stay vector. Encrypted PDFs are opened with the restriction ignored; whether that works depends on the encryption.