· 2 min read· by Dilpreet Grover
Redacting scanned documents: the text layer you can't see
Scanned PDFs often hide a searchable text layer behind the image. Blacking out the picture leaves that text behind. How to check a scan, and how to redact it.
Creator, Ombly
A scanned document feels safe to redact. It's just a picture, so blacking out part of the picture should hide it. Often, though, there's more to a scan than the picture.
Why scans have hidden text
Most scanners, copiers and phone scanning apps run optical character recognition (OCR) on what they capture. OCR reads the words in the image and stores them as an invisible layer of text behind the picture. That's what makes a scanned PDF searchable, and it's usually switched on by default.
The result is a page that looks like an image but contains the full text too, lined up behind each word.
How redaction goes wrong
If you draw a black box over the image, you cover the picture of the account number. The invisible text layer underneath still holds the number. Anyone who selects and copies the page, or searches the file, can get it back.
The reverse can happen too: a tool might remove the text layer but leave the picture, or the text layer may not line up exactly with the image, so a redaction misses part of a word.
How to check a scan
- Try to select text. If you can highlight words on a scanned page, it has a text layer.
- Search for something you hid, such as the last four digits of an account number.
- Copy and paste the whole page into a plain text note and read it.
- Zoom in on the edges of each black box to make sure nothing peeks out.
How to redact a scan properly
- Use a tool that redacts both, removing the words from the text layer and painting over the image.
- Or flatten it. Rebuild the page as a plain image after redacting, so there's no text layer left at all. The trade-off is that the result is no longer searchable.
- Check the result using the steps above, every time.
For the full routine, see how to redact a PDF properly, and for why covering isn't enough, a black box is not a redaction.
Where Ombly fits
Ombly reads scans with OCR on your own computer to find what to hide, then rebuilds every page from pixels. The export has no text layer at all, so there's nothing behind the picture to recover. Join the waitlist to try it first.
Keep reading
· 2 min read
A black box is not a redaction
Drawing a black box over an account number in a PDF often leaves the text right underneath. Here's why that happens, and what real redaction looks like.
Read →· 3 min read
How to redact a PDF properly (and check that it worked)
A practical way to hide account numbers, IDs and names in a PDF so they're actually gone, plus a two-minute check that catches the most common mistakes.
Read →Share the document. Keep the secrets.
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