Add watermark to PDF

This tool draws one line of bold text across the center of every page in your PDF. The text is centered horizontally and sits at the vertical midpoint of each page, so a single stamp covers the heart of the content rather than tiling repeatedly across the whole sheet. It is rendered in Helvetica Bold in a dark gray, with the see-through effect coming from the opacity you set.

The font size is calculated automatically from each page's width, scaling between 18 and 54 points. That means a wide landscape page gets a proportionally larger stamp than a narrow portrait one, and a mixed-orientation document stays visually consistent. You control the text, the opacity, and the rotation angle (-45, 0, or 45 degrees); the placement and sizing are handled for you.

Because the stamp is a single centered string, very long phrases may extend toward the page edges. Short labels such as DRAFT, CONFIDENTIAL, or an organization name read most cleanly. Watermark text is capped at 80 characters, and control characters and extra spaces are collapsed before the text is drawn.

Opacity is deliberately limited to a range of 5 to 35 percent, with 15 percent as the default. The cap exists so the watermark never fully obscures the underlying text, tables, or figures. A setting of 10 to 20 percent usually keeps the stamp clearly visible while leaving the document comfortable to read; push toward 35 percent only when you specifically want the mark to dominate.

The angle options are -45, 0, and 45 degrees. A diagonal angle (the -45 default) is the conventional look for status stamps like DRAFT and tends to overlap less awkwardly with horizontal body text. A flat 0-degree angle can work for a centered name or label, though it sits directly across the middle line of running text.

If the result looks too faint or too heavy, there is no in-place adjustment: run the tool again on the original file with a different opacity or angle. Keep using the unwatermarked source each time rather than re-stamping an already-marked copy, which would layer two stamps.

This tool runs entirely in your browser. Your PDF is read, stamped, and saved on your own device using an in-page PDF library, and the file is never uploaded to a server. The generated download is held in memory and the temporary link is cleaned up automatically after about ten minutes, so reload the page if you need to start over.

The input must be a valid, unencrypted PDF. Password-protected files cannot be modified and will be rejected with a message; remove the protection first if you own the document and have the right to do so. Use this tool only on files you have the rights to alter.

The watermark is written into the PDF's content layer, which makes it part of the page rather than a removable annotation. FormatOS does not offer a watermark-removal tool, so keep a copy of your original unwatermarked file. That clean copy is your route back if you later need a version without the stamp or want to re-stamp with different settings.