BASALT · JOURNAL
Adding a watermark to a PDF on a Mac that survives the print
You mark a draft CONFIDENTIAL, send it out, and someone prints a copy where the word is not there. The watermark was on screen, so what happened. PDF offers two completely different places to put a mark, and only one is part of the page.
Annotation or page content
A watermark drawn as page content is written into the page's content stream: the same sequence of operators that draws the body text and the logo. It is part of the page in the sense a paragraph is. It prints, it survives flattening and re-saving, and removing it means editing the page itself.
A watermark added as an annotation is a separate object listed in the page's /Annots array with its own appearance stream. Whether it prints depends on the /Print bit in the annotation's /F flags field, which many tools leave clear because annotations were designed for review markup you keep off paper. Any PDF editor, Preview included, deletes it in one action.
That is the honest tradeoff. Content is permanent and prints. An annotation can be deleted and may not print. If your watermark exists to tell a reader something on paper, put it in the content stream. If it is a review sticker meant to stay off the printed copy, an annotation is correct and not a bug.
Neither form is access control. Both can be removed by a determined recipient.
What a watermark is made of
A well-formed text watermark is a block of operators wrapped in q and Q to save and restore the graphics state, so it neither inherits nor leaks a clipping path, a color, or a transformation matrix. Inside, it sets a matrix for position and rotation, selects a font resource, sets a fill color, and emits a text-showing operator such as Tj.
Transparency comes from an /ExtGState resource setting the fill alpha, typically between 0.1 and 0.3. A multiply blend mode helps a watermark read consistently over both white space and dark graphics, but blend modes only behave predictably inside a transparency group, so plain alpha is the portable choice.
Placement is where naive scripts fail. Compute the center from the page's CropBox when one exists, falling back to the MediaBox, because the CropBox is what a viewer displays. Then account for /Rotate, which rotates the rendered page without rotating the content coordinate system. Any position hardcoded in points will be wrong on a document mixing letter, legal, and scanned pages.
Under the content or over it
You can prepend the watermark operators so it draws first and the page text lands on top, or append them so it draws last and sits over everything.
Underlaying looks better on text-heavy documents because the body copy stays legible. It fails on scanned pages, because a scan is one opaque image covering the whole page and anything beneath it is invisible. Overlaying always shows but reduces legibility of whatever it crosses. Overlay on scanned pages, underlay on born-digital text, and when a document mixes both, overlay with a low alpha and check.
Headers and footers are the same mechanism
A running header, a footer, and a Bates number are the same operation with different geometry: text placed in a margin computed from each page's own box. The concern shifts from legibility to collision with existing content.
Keep them in the content stream for the same reason. A footer carrying a matter number is worthless if a downstream tool can strip it silently. That argument also governs Bates numbering on a Mac, where a deletable identifier defeats the purpose of the number.
What macOS gives you
Preview has no watermark feature. A markup text box approximates one, but it creates a free text annotation on a single page: deletable, print-flag dependent, and impractical to place by hand across a long document.
Quartz filters transform rendering rather than compose overlay text, so they are the wrong tool. Automator's PDF actions can overlay one PDF onto another, which does write into the page, but the overlay position is fixed rather than computed per page, so it drifts on mixed sizes and rotated pages. The command line route is the same idea done properly: generate a per-page overlay sized from each source page's CropBox and /Rotate, then merge with pdftk-java multistamp. The geometry is the work.
Order of operations
Watermark before you redact, not after. If you redact first and watermark second, the watermarking pass rewrites every page's content stream, and you then have to prove that rewrite preserved the redaction. Watermark first and the redaction pass is the final content operation, so its verification covers the finished file. Check the printed output: alpha, blend modes, and print flags all behave differently on paper.
Frequently asked questions
How do I add a watermark to a PDF on a Mac?
Use a PDF application that writes the watermark into each page's content stream and computes the position from the page's own CropBox and /Rotate value. macOS has no built-in watermark feature: Preview only inserts free text annotations, which any editor can delete and which may not print at all.
Why does my watermark not print?
Because it is an annotation whose print flag is not set. Annotations carry a flags field where one bit controls whether the object appears in printed output, and tools built for review markup often leave it clear on purpose. A watermark written into the page content stream always prints, because it is part of the page.
Can a watermark be removed from a PDF?
Yes, by anyone with the right software, regardless of how it was added. An annotation watermark is deleted in a single action, and a content stream watermark can be removed by editing the page's operators, which takes more effort but is entirely possible. A watermark communicates provenance; it is not a security control.
Should a watermark go over or under the page content?
Overlay on scanned or image-heavy pages and underlay on born-digital text pages. A scan is one opaque image covering the whole page, so anything drawn beneath it is invisible, which is why underlaid watermarks disappear on scanned documents. When a document mixes both kinds, overlay with a low alpha and inspect the result.
Does Preview have a watermark feature?
No. Preview can insert a text box with its markup tools, but that produces a free text annotation on one page rather than a watermark composed into the page content across the document. It is deletable by the recipient, its printing depends on annotation flags, and placing it by hand on every page of a long file is not workable.
How do I watermark only some pages?
Use a tool that accepts a page range and applies the watermark operators only to pages in that range. This matters for productions where a confidentiality designation covers specific pages rather than the whole document. If the same selection recurs across files, save it as a repeatable procedure, as described in running the same PDF steps repeatedly.
Why is my watermark sideways on some pages?
Those pages almost certainly carry a /Rotate value of 90 or 270. That key rotates the page as displayed without rotating the content coordinate system, so text placed with an assumed orientation reads correctly in content space and sideways to the reader. Apply the inverse rotation to the watermark's matrix, per page.
Does watermarking a PDF change the original file?
It depends entirely on the tool. Some applications rewrite the file in place, so a mistake in the watermark costs you the source document. Tools that write a new output file leave the original intact, which is safer when watermarking is one step in a pipeline that also includes redaction and numbering.
Doing it in Basalt
Basalt's WATERMARK tool handles watermarks, headers, and footers in one place. Every tool writes a new file, so your original is never modified and a bad watermark costs you nothing. Everything runs inside a sealed engine process holding no network entitlement, enforced by macOS at the code-signature level, so documents never leave the Mac: no account, no telemetry, no cloud. Watermarking is one of 18 tools in the same window, alongside redact, Bates numbering, and verification, for $29 once across up to three Macs, free for 24 hours, at basaltformac.com.
Redaction that proves itself
Basalt destroys the content you mark, then re-opens the file it wrote and proves the content is gone before it saves anything. Your documents never leave your Mac.