How PDF text watermarks work and their limits
What an in-browser text watermark does to each page, how opacity, size and angle affect it, and why marking a PDF is not the same as securing it.
What the stamp does to each page
A text watermark is not a separate overlay floating above your document; it is real page content painted directly into the PDF. This tool loads the file with pdf-lib, embeds the Helvetica Bold font, and for every page measures the width of your text and draws it centered, tinted and semi-transparent. Because the drawing happens in the page's own coordinate space, the stamp lands correctly even on pages that were rotated, and it becomes part of the document you download rather than a viewer setting that only you can see.
Opacity, size, color and angle
Four controls shape the look. Opacity, from 10% to 60%, decides how much of the underlying text shows through, with lower values acting as a faint background and higher ones as a bold overlay. Font size in points sets how large the word is, clamped to a readable 8 to 200 range. Color takes a hex value and defaults to a neutral gray so the mark reads without hiding content. Angle offers a flat 0 degree line or the classic 45 degree diagonal, which sweeps across more of the page and is harder to crop away.
Marking is not securing
It is worth being honest about what a watermark can and cannot do. Stamping CONFIDENTIAL or DRAFT communicates intent and discourages casual misuse, and because the text is baked into the content stream it will not simply toggle off. But it is not encryption and not a lock: a determined person with the right software can still edit the page, and the words sit over the content rather than restricting who can open the file. If you need to actually prevent access, pair the mark with a password or a rights-managed export from dedicated software.
Why in-browser processing matters
The entire operation runs as client-side JavaScript, which has a real privacy payoff. Your PDF is read from disk into the browser's memory, stamped, and offered back as a download, and at no point is it transmitted to a server. That means sensitive contracts, medical records and financial statements never touch someone else's infrastructure, there is nothing to delete from a cloud queue afterward, and the tool keeps working even with your network disconnected once the page has loaded.