Introduction
The PDF format is widely used to share sensitive documents such as payslips, bank statements, invoices, or proof of address. Its popularity comes from its visual stability and universal compatibility. However, this apparent reliability can be exploited by fraudsters who use techniques invisible to the naked eye to alter a document without changing its overall appearance.
Among these techniques, the use of overlays is one of the most common and effective methods. An overlay makes it possible to layer new elements on top of the existing content of a PDF, in order to hide or change certain information without deleting the original content. This technique is particularly difficult to detect visually, but it can be uncovered through an in-depth forensic analysis of the file’s structure.
Understanding how overlays work and how to detect them is essential to secure document verification processes and prevent fraud.
What is an overlay in a PDF?
An overlay in a PDF is a graphical or text element added on top of the document’s existing content. It does not necessarily replace the original content, but visually covers it. The initial content can therefore remain present in the file, even if it is no longer visible during normal viewing.
From a technical standpoint, a PDF file is made up of a set of objects organized and rendered in a specific order. Each object can contain text, images, shapes, or drawing instructions. When an overlay is added, a new object is simply inserted into the PDF structure, with a position and rendering order that places it above the existing objects.
This means the original content is not necessarily removed. It is simply hidden by a new object that covers it.
This characteristic makes overlays particularly dangerous in the context of document fraud, because a document can appear perfectly authentic while containing changes that are invisible to the naked eye.
Why overlays are used to falsify documents
Overlays are commonly used to alter sensitive information in official or financial documents. For example, a fraudster may use an overlay to change a salary amount on a payslip, increase the balance shown on a bank statement, or modify an issue date on a supporting document.
In these cases, the fraudster simply adds a graphical object, often a white rectangle, to hide the original value. Then they add a new text object on top, containing the falsified value. Visually, the document looks consistent and authentic, because the displayed content matches what the user expects to see.
This technique offers several advantages for fraudsters. It is quick to implement, does not require deeply modifying the original document, and does not degrade the file’s visual quality. Unlike image editing or a complete reconstruction of the document, an overlay preserves the professional appearance of the PDF.
In addition, because the original content often remains present in the file’s structure, the document can pass simple visual checks without raising suspicion.
How an overlay is technically added to a PDF
Adding an overlay relies on the internal workings of the PDF format. A PDF contains a list of objects that describe the document’s visual elements and their positions. When software edits a PDF, it can add new objects without modifying the existing ones.
These new objects can contain text, images, or shapes. They are placed at precise coordinates and inserted into the document’s rendering order. Because the PDF rendering engine displays objects in that order, the most recently added objects appear on top of the existing ones.
Some software also uses incremental updates, meaning changes are appended to the end of the file without modifying the original data. This method leaves a technical trace of the modifications, but it does not change the document’s visual appearance.
This makes it possible to alter a PDF without recreating the entire file, which makes the forgery faster and more discreet.
Why overlays are invisible to the naked eye
Overlays are invisible to the naked eye because the PDF viewer only shows the final rendered result. It does not display the file’s internal structure or any hidden objects.
When a white rectangle is placed on top of existing text, it completely covers that text. Then a new text element can be added on top, creating the impression that the document contains only the new value.
For a user or a human reviewer, the document looks perfectly normal. There are no abnormal pixels, no visual artifacts, and no visible indication that a modification has been made.
The only way to detect an overlay is to analyze the internal structure of the PDF file.
Individuals, do you occasionally need to verify one or more identity documents ?
Professionals, do you need to integrate proof of identity validation into one of your business processes, including all European identity cards and global passports ?
How to detect an overlay using forensic analysis
Overlay detection relies on a technical analysis of the PDF file. Unlike a simple visual inspection, forensic analysis examines the document’s internal structure, its objects, its content streams, and its modification history.
One of the most effective methods is to analyze the objects present in the document and their rendering order. If a text or graphic object was added after the fact and covers another object, this may indicate the presence of an overlay.
It is also possible to analyze the PDF’s incremental updates. These updates make it possible to identify objects added after the document’s initial creation. The presence of objects added later in critical positions can reveal an attempt to modify the document.
Metadata analysis can also provide important clues. For example, if the document was created with one specific software tool but later modified with a different PDF editor, this may indicate a potentially suspicious change.
Finally, some techniques make it possible to reconstruct the document’s original content by ignoring the added objects. This can reveal information hidden by overlays.
Concrete example of an overlay-based forgery
Let’s take the example of a payslip. The original document contains a net salary of 1,800 euros. A fraudster wants to increase this amount to 2,800 euros in order to obtain a loan.
The fraudster opens the PDF in an editor and adds a white rectangle over the original amount. This rectangle visually hides the existing value. Then they add new text displaying the falsified amount.
The document appears perfectly authentic. The displayed amount is consistent with the rest of the document, and no visual defect is apparent.
However, a forensic analysis reveals the presence of new objects added after the document’s initial creation. The analysis of the internal structure also makes it possible to retrieve the original amount, which is still present in the file.
Difference between an overlay and a real modification of the document
It is important to distinguish overlays from real modifications of the document. In the case of an overlay, the original content is generally preserved and simply hidden. In the case of a real modification, the original content is replaced or deleted.
Overlays are often easier to detect during a forensic analysis because they leave technical traces in the PDF structure. By contrast, some advanced modifications can recreate the entire document, which makes detection more complex.
In all cases, technical analysis of the file remains the most reliable method to detect forgeries.
Why overlay detection is essential
Overlay detection is essential in many contexts, including KYC processes, credit applications, identity verification, and fraud prevention.
PDF documents are often considered trustworthy, but they can be edited easily using tools available to the general public. Without forensic analysis, it is impossible to guarantee the integrity of a PDF document.
The ability to detect overlays makes it possible to verify whether a document has been modified, even when the modification is visually invisible. This strengthens verification processes and reduces fraud risk.
How TrustDocHub detects overlays in PDFs
TrustDocHub uses advanced forensic analysis to examine the internal structure of PDF files. This analysis makes it possible to identify added objects, structural inconsistencies, and incremental modifications.
The system analyzes the rendering order of objects, content streams, and the document’s metadata to detect any suspicious modification. It is also able to identify overlapping objects and hidden elements.
This analysis makes it possible to detect overlays, even when the document appears perfectly authentic visually.
Conclusion
Overlays represent a particularly effective fraud technique, because they allow the appearance of a document to be altered without directly modifying its original content. These changes are invisible to the naked eye and can go unnoticed during a visual check.
However, overlays leave technical traces in the internal structure of the PDF file. A forensic analysis makes it possible to identify these traces and detect modifications.
In a context where digital documents play a central role in verification processes, overlay detection is essential to ensure the authenticity and integrity of PDF files.
Individuals, do you occasionally need to verify one or more identity documents ?
Professionals, do you need to integrate proof of identity validation into one of your business processes, including all European identity cards and global passports ?
