Stamp locally: when a mark helps, and when it just annoys readers
A watermark is a repeated overlay — usually diagonal DRAFT or CONFIDENTIAL, sometimes a faint logo — so anyone with a copy knows the status. Convertify4u draws that overlay with pdf-lib inside a Web Worker. The PDF never leaves your machine on this Instant path. That is the opposite of upload-and-auto-delete converters: there is nothing to delete because nothing arrived. Everyday single files of normal size are free. Multi-file queues and documents over about 100MB need Premium (Lemon Squeezy) because browser memory is finite. Too faint (under ~15% opacity) disappears on a laser printer; too heavy (over ~60%) makes tables unreadable. 20–35% with a slight rotation is the usual compromise. This is not DRM and not encryption — anyone can crop or screenshot. It is a social and policy signal. Vector page content stays vector; we overlay rather than print-to-PDF rasterizing the whole document. Open spokes for text presets, PNG logos, business packs, batch, free/no-signup, mobile, or extra-private handling notes.
How stamping works in this tab
Drop a PDF. Choose text or a PNG. Set opacity, rotation, and a grid cell (corners, center, tiled). pdf-lib writes the overlay onto each page in a Worker thread so the UI stays responsive. Download the new file. Keep the unmarked original — you cannot “undo” a stamp on a copy you already emailed.
Instant versus Premium
A typical contract or invoice stamps for free with no account. Folder-scale work and huge scanned archives need Premium so a single tab is not paging gigabytes. We say that up front rather than failing at 98%.
What to do after the stamp
Compress if email caps bite. Merge if you stamped parts separately. Do not confuse a visible mark with Protect PDF (passwords) or Redact (removing text). Those are different tools.