How it works
Every copy of a document can be made different
A company that shares a sensitive document can give each person a copy with its own invisible marks: a slightly wider space, a dot pattern too pale to see, a character that looks like every other. If a copy turns up somewhere it shouldn't, the marks show whose copy it was.
Below, each kind of mark is shown exaggerated so you can see it, with a slider to bring it back down to how it really looks. Then you'll see what Unmark PDF removes, what the Claude rewrite adds, and what nothing can remove.
When you open a document, Unmark PDF first checks it for signs of these marks and tells you what it found. It cleans the document either way.
1 · SpacingRemoved by the clean copy
Spaces that are a hair too wide
A PDF places every word at an exact position, so the gap after each word can be nudged by a fraction of a point. Wide gap means 1, normal gap means 0. Read the gaps in order and you get a number that's different for every copy.
The same trick works on line spacing, letter spacing and tiny shifts up or down of single letters. Printed and scanned again, the marks are still there.
What Unmark PDF does: it never copies the layout. The words are taken out and set again in a new PDF with standard, even spacing, so every gap is the same. It can follow the original's headings, bold words, lists and tables, but sets them from scratch in standard fonts. After creating the PDF, it reads the file back and measures every word gap and line to confirm it.
2 · Faint shadingRemoved by the clean copy
A dot pattern too pale to see
A background of tiny dots, only a shade off white, can be printed behind the text. Many color printers already add yellow tracking dots that record the printer's serial number and the time. A document can carry its own pattern the same way, repeated across the page so even a torn corner holds a full copy of it.
Second quarter forecast. Revenue in the northern region is expected to rise 6 percent, mostly from the two new service contracts signed in April. Costs stay flat apart from the planned office move in August.
Staffing stays at 42 people. The hiring freeze will be reviewed at the September board meeting, and any exceptions need sign-off from the finance director.
One tile of the pattern, enlarged. The solid corner shows which way is up; the other dots spell out the copy number in binary: copy 45.
What Unmark PDF does: images, backgrounds and shading are never carried over. Only the words go into the new PDF, on a plain white page.
3 · The text you selectRemoved by the clean copy
What you copy isn't always what you see
A PDF keeps the picture of the page and the text you can select as two separate things, and they don't have to match. When you select, copy or search, you get the hidden text layer, which can carry its own marks:
- Invisible characters between letters, such as zero-width spaces. Two kinds stand for 0 and 1.
- Look-alike letters from another alphabet. A Cyrillic “а” looks the same as a Latin “a” but is a different character.
- Hidden words in white or behind other text, such as a reference number.
Try it: select the sentence above and paste it into a document or a character counter. The hidden characters come along with it.
What Unmark PDF does: it removes invisible characters, replaces look-alike letters with the normal ones, and reports how many it found. For PDFs, “Only what's visible on the page” reads what's drawn instead of the hidden text layer, so white or hidden words are left behind.
4 · WordingRemoved by the Claude rewrite
The words themselves can be the mark
The strongest marks live in the wording. Each copy uses slightly different word choices, word order or punctuation. Nothing is hidden, so cleaning characters and rebuilding the PDF can't find anything to remove. Anyone who quotes or pastes the text carries the mark with it.
Copy for recipient A
Copy for recipient B
What the Claude rewrite does: it says the same thing in new words and new sentence structure, so the word choices that told the copies apart are gone. What's left matches no copy any better than another.
5 · Summary
What each step removes
| Kind of mark | Clean copy | + Claude rewrite |
|---|---|---|
| Author, company and edit history in the file | Removed | Removed |
| Spacing, line and letter positions | Removed | Removed |
| Faint shading, dot patterns, images | Removed | Removed |
| Invisible characters and look-alike letters | Removed | Removed |
| Hidden text in a PDF | Removed with “Only what's visible” | Removed with “Only what's visible” |
| Word choice, word order, punctuation | Kept | Removed |
| Facts that differ between copies (numbers, dates, names) | Kept | Kept |
6 · About the Claude rewrite
The rewrite points to Claude, not to you
Claude's output carries Anthropic's statistical watermark. It shows that Claude was involved in writing the text, not who used it. So the old marks that pointed to a particular copy are replaced by one that only says an AI wrote the words.
- The rewrite is sent through this site's own Anthropic account, not one of yours.
- This site doesn't keep your text. Anthropic receives it to do the rewrite and handles it under its API terms.
- Only the text in the editor is sent. Your original file never leaves your computer.
7 · What nothing can remove
Facts stay facts
The rewrite keeps every number, date, amount and name exactly as written; that's what makes it safe to use. It also means a difference planted in the facts survives. If one copy says the budget is $48,200 and another says $48,300, the rewrite keeps whichever one you had.
The content can point to a source too: a detail only a few people knew is still there after any amount of cleaning. No tool can promise a document can never be traced. Unmark PDF removes the hidden marks; read the result before you share it.