The data hiding in your photos and PDFs, and how to remove it

Every photograph your phone takes carries a second, invisible document attached to it. So does almost every PDF you produce. Neither is displayed when the file is opened, which is precisely why people share them without thinking.

Most of the time this is harmless. Occasionally it is not, and the difference is worth being able to judge.

What a photo actually carries

The technical name is EXIF, and it was designed for good reasons: photographers wanted to know what settings produced a particular shot. The format grew, phones adopted it, and it now records a great deal more than exposure settings.

• GPS coordinates, often accurate to a few metres
• Date and time to the second, including time zone
• Device make and model, and sometimes a unique device identifier
• Camera settings, orientation and lens information
• Editing software used, and occasionally a thumbnail of the original image before cropping

That last item deserves attention. Some editors store a preview of the image as it was before you edited it. Crop someone out of a photograph and the embedded thumbnail can still show them, because the thumbnail was never regenerated.

The coordinates are the part that matters most often. A photo taken indoors carries the location of that room. Sell an item online with photographs taken at home, and the listing can carry your address to anyone who knows where to look.

What a PDF carries

Documents are less dramatic but more professionally awkward. A PDF exported from a word processor typically records the author name as configured on that computer, the organisation, the creating application and version, and creation and modification timestamps.

The author field is the one that causes trouble. It is set once when the software is installed and then forgotten, so it frequently holds a personal name, a previous employer, or the name of whoever originally created the template the document was built from. Sending a proposal that names a competitor as its author is a memorable way to discover this.

Timestamps tell their own story too. A document dated as submitted on Friday but modified on Sunday evening is a detail that has come up in more than one dispute.

When it genuinely matters

Being proportionate is more useful than being paranoid. Metadata is worth removing when the file is going somewhere you do not control or to people you do not know.

• Photographs on marketplace listings, forums, or anywhere public
• Images of your home, workplace, children or vehicle
• Documents sent to opposing parties in a dispute, or to a competitor
• Anything submitted anonymously, where the author field defeats the anonymity
• Files published on a website, which are downloadable by anyone with metadata intact

Conversely, there is little reason to strip metadata from a holiday photo sent to a family member, or an internal document circulating among colleagues who already know who wrote it. Timestamps and authorship are frequently useful inside an organisation.

What social platforms do and do not do

Most large social networks strip EXIF from uploaded images, partly for privacy and partly because it saves storage. This has convinced a lot of people that the problem is handled.

It is handled only on those platforms. Messaging apps vary, some preserving the original file entirely when sent as a document rather than a photo. Email attachments are untouched. Files uploaded to forums, listing sites, cloud links and content management systems generally keep everything. And a platform stripping metadata on display does not mean it discarded the data on receipt.

Removing it

1. Strip metadata as the last step, after all editing is finished, since editors can write new metadata as they save.
2. Process images before uploading them anywhere public, not afterwards.
3. For documents, check the properties panel of your PDF reader to see what the author and title fields actually contain.
4. Re-check one file after processing to confirm the fields are genuinely empty.
5. Where a document is highly sensitive, consider flattening it to images, which discards structure along with the metadata.

One habit is worth building above all the rest: look at the properties of a document before you send it externally, the same way you would check the recipient line. It takes a moment and it catches the author-field problem before anyone else sees it.