PDF privacy
Text no one can see in a PDF
Text in a PDF does not have to be visible to be there. Written in white on a white page, drawn with no ink, placed beyond the page's edge or made a fraction of a point high, it is invisible to a reader — and read in full by search, by copy and paste, by screen readers, and by the programs that screen CVs and documents. hexscope finds it and shows you what it says.
Check a file Try a PDF with a hidden note
1 Four ways to hide text
- White on white. White text with nothing but the page behind it. White text on a dark band is a design, and hexscope leaves it alone.
- No ink. Text rendering mode 3 draws text invisibly. Scanned pages use it for their recognised words, so a page that is a scan does not count.
- Off the page. Placed outside the page's area, where no viewer shows it and no printer prints it.
- Too small. Under a point high: a speck, or nothing, on the page.
2 Why it matters
It can be an honest leftover — a note from an earlier draft, a template's placeholder. It can also be a message for software: words in a CV meant for the screening program and not for the person, such as instructions addressed to an AI. Either way, whoever gets the file can read it.
A black box drawn over text is the same problem the other way round, and so is a black highlight — a name marked in black, in a PDF made from a web page or a word processor, is black text on a black box. The text is hidden from the reader and there for everyone else. hexscope finds all of them, and the clean copy takes them out.
3 Find it
Open the PDF in hexscope. Hidden text appears under What this document reveals as Hidden text, page by page, with how it is hidden and what it says, and the verdict opens on Something is hidden or disguised. Each finding links to the page content that holds it.
Text in a font whose codes do not map to letters is counted rather than guessed. hexscope follows nested form XObjects using their own resources, but marks a page incomplete when a form cannot be fully read or safely isolated; in that case it will not make a PDF copy that could leave hidden text behind.
4 Remove it
Remove it — save a clean copy takes hidden text out of the page, letter by letter, and leaves everything else where it was. Open the clean copy to check nothing is left.
If you received the file, removing the text does not tell you who put it there — ask. If you are sending it, check it first: a template or an export can leave text you never saw.
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Text rendering modes are in ISO 32000-1, section 9.3.6.