The free check looks in ten places where words can hide inside a PDF (Portable Document Format) file. Here is each one in plain words. The check only reads your file, on your own phone or computer. Your file stays on your device.
A clean result means these checks found nothing. It does not mean the file is certified safe. A warning means something can still be read, so look for yourself. It does not mean anybody hid it on purpose.
The short answer. The free check looks in ten places where words can hide inside a PDF file, from words under a black box to invisible text, earlier versions, document properties and files tucked inside. A clean result means the checks found nothing. It does not mean the file is certified safe.
Words under a black box or a pasted picture
The most common mistake. Somebody draws a black box, or pastes a black picture, on top of the words. The page looks right, but the words are still in the file underneath, and anyone can copy them out.
The check looks at where every letter sits on the page and asks whether something solid was drawn on top of it afterwards. A picture only counts as a cover if it was drawn after the words, and if the part over the words is one solid colour. Black, white or grey all count: a white strip over words hides them just as well. A colourful logo that happens to overlap some printing does not count.
If it is sure, it says the words can be recovered. If a picture sits over the words but the check cannot be sure it hides them, it says possible and asks you to look for yourself. It never passes that case quietly.
Invisible words
Words can be set to be invisible, or printed in the same colour as the page, such as white on white. You cannot see them, but they are in the file. The check looks for both.
A hidden copy of the words behind a scan
When a scanner or phone app reads the words on a scanned page, it often hides an invisible copy of them behind the picture so the document can be searched. Black out the picture and that hidden copy stays. The check looks for scanned pages that still carry one.
Boxes that are only notes on top
Some programs add the black box as a note or highlight sitting on top of the page, like a sticky note, and never remove the words underneath. The check lists any notes of that kind.
Earlier versions inside the file
Many programs save changes by adding them to the end of the file instead of rewriting it. The older version, words and all, can still be inside. The check counts how many times the file was saved this way.
Document properties
Every PDF file can carry a hidden label card: author, title, subject, keywords, the program that made it, and dates. The check lists what it finds and points out anything that looks like a person’s name, a folder on somebody’s computer, or a client’s name.
Files tucked inside
A PDF file can carry other files inside it, like an email carries attachments. The check lists any it finds, with their names.
Filled-in forms
Answers typed into a form are kept in the form boxes, even when the page looks blank. The check lists the boxes and what is in them.
Hidden programs
A PDF file can carry small programs that run when it is opened. The check tells you if there are any. It never runs them.
The side menu and page names
The list of sections down the side of a PDF file, and custom page names, have their own words. They sometimes still name a person or a case. The check lists them.
What the results mean
The hidden words can still be read. — Words are still in the file: under a box, under a covering picture, printed invisibly, or hidden behind a scanned page. Do not send it. Go back to the original and black it out again with something that removes the words.
Nothing was found under the marks — but something else in this file can still be read. — The boxes held, but one of the other places above holds something, or a picture over the words could not be proved solid. The list tells you which.
No problems found by these checks. — Every check passed. It is not a safety certificate: look at every page yourself as well.
Every page is a picture, so there are no hidden words to get back out. — That is what a properly blacked-out document looks like when each page has been turned into a picture. The check cannot see what the pictures show, so look at them with your own eyes.