Mark a PDF as confidential
A diagonal CONFIDENTIAL on every page, added on your device.
- Nothing is uploaded
- Works offline after first visit
- Open source
- The limit is your device’s memory (phones hold less)
Stamp CONFIDENTIAL diagonally across every page of a PDF, in red or any colour, in your browser. It is free, and your files are not uploaded: everything runs in your browser on your device. The limit is your device’s memory (phones hold less)
Done
Open the network tab. Your files never appear in it.
Add files and run the tool to see every request this tab makes.
Your files are processed on this device and never sent to a server. Reload the page to clear everything from memory.
A CONFIDENTIAL watermark tells every reader, on every page, that the document is not for passing around: a draft contract, board papers, a salary review, a due-diligence pack, a patient letter. It does not stop anyone from sharing it, but it removes any doubt about whether they were allowed to, and a leaked page is recognisable at a glance.
This page puts the word diagonally across the middle of each page, faint enough that the text underneath stays readable, and sized to each page, so a landscape spreadsheet printout gets the same mark as the A4 letter before it. You can change the wording to STRICTLY CONFIDENTIAL, INTERNAL ONLY, or a person’s name and date to trace a copy back to who received it.
The irony of most online watermark tools is that they ask you to upload the confidential document to mark it confidential. Here the PDF is opened and stamped inside your browser tab, and the file is not uploaded.
How it works
- Drop one or more PDFs, or tap to pick them.
- Keep CONFIDENTIAL or type your own wording, and check the preview of page 1. Lower the opacity if the text underneath is hard to read.
- Download the marked PDF; every page carries the watermark.
In your browser, not on a server
pdf-lib (MIT) opens the PDF in your tab. For each page size, the watermark is drawn once on a transparent canvas at about 216 dpi and embedded as a PNG, then placed over every page of that size, following each page’s crop box and rotation. The original page content is not rewritten, so text, links and forms are untouched.
Compared with upload-based tools: Online PDF watermark tools upload the whole document to add a line of text to it, and the documents people watermark are the confidential ones. Here your files are not uploaded, there is no page or file limit, and nothing but your watermark is added.
Verify it yourself. Give the page a short while after one visit to save itself for offline use, then turn off Wi-Fi or switch to airplane mode and run the tool again. Or open your browser's developer tools (F12, or Cmd-Option-I on a Mac), pick the Network tab, and watch it while you convert. The panel above does the same count for you.
Questions
Can I add the recipient’s name?
Yes, and it is a good idea for sensitive documents: type something like “CONFIDENTIAL · Prepared for J. Smith · 3 Oct 2026”. If a copy turns up somewhere it should not, you know whose it was. Run it once per recipient.
Will it still print well?
Yes. The watermark is drawn at about 216 dpi, sharp on paper, and at 20% opacity it prints as a light tint that does not hide the text. Red and grey both photocopy clearly.
Is a watermark enough to protect a confidential document?
It marks it, it does not lock it. To stop people opening it without permission, also add a password with the protect PDF tool, and send the password by a different channel than the file.
Can I mark only some pages?
This page marks every page. To mark part of a document, split out those pages with the split PDF tool, watermark them, and merge the parts back together.
Is the file uploaded anywhere?
No. There is no server in this process. The page is a static file and the work is done by code running inside your browser tab. Give the page a short while after one visit to save itself for offline use; then you can turn off Wi-Fi and it still works.
More PDF tools
Watermark PDF
Mark every page as a draft, confidential or a copy for one person, without sending the document anywhere.
Remove PDF metadata
See the name, software and dates hidden in a PDF, and strip them before it goes out.
Grayscale PDF
Every colour becomes its own shade of grey, and the text stays text.
Crop PDF
Cut PDF pages down to what matters: draw the area to keep, or trim the white margins automatically.
Other conversions
Guides
- How to add a watermark to a PDFStamping "confidential" or "draft" across every page needs either the original document or a tool built for it. Here is how.
- A free Smallpdf alternative with no daily limitSmallpdf's free plan has daily limits. Here is what that means, and a way around it where your files are never uploaded.
- A free iLovePDF alternative with no task limits or adsiLovePDF's free plan works, but with limits on documents, batches and file size. Here is what changes with a tool that never uploads the file at all.