Introduction: why incremental updates are a key element of PDF forensic analysis
The PDF format is now ubiquitous in professional exchanges. It is used to transmit sensitive documents such as invoices, bank statements, payslips, contracts, or identity verification documents. Its popularity is based on its universal compatibility and its ability to preserve the visual appearance of documents. However, this apparent reliability hides a more complex technical reality.
Contrary to what one might think, a PDF file can be modified without its original content actually being removed. Thanks to a mechanism called an incremental update, it is possible to add new versions of certain elements of the document without rewriting the entire file. This behavior, perfectly normal and defined by the PDF specification, is also exploited in many cases of document fraud.
Forensic analysis of incremental updates is therefore an essential step in detecting falsifications invisible to the naked eye and verifying the true authenticity of a PDF document.
What is an incremental update in a PDF file
An incremental update is a mechanism that allows a PDF file to be modified by simply adding new information to the end of the existing file, without modifying or deleting the original data. Instead of rewriting the entire document, the software adds a new version of the modified objects, along with a new reference table and a new internal pointer.
In practical terms, this means that when a PDF is modified, the previous version of the content remains physically present in the file. The new version is simply appended afterward, and PDF readers use internal pointers to display the most recent version.
This behavior makes it possible to preserve a technical history of modifications, but this history is not visible in standard PDF readers.
Why the PDF format allows incremental updates
The PDF format is based on an internal architecture composed of objects, reference tables, and structures called trailers. Each element of the document, whether text, images, or metadata, is stored as objects identified by unique numbers.
A table called xref indicates the position of each object within the file. A trailer, located at the end of the document, contains the information necessary to locate this table.
When a modification is made, the software does not need to modify the existing objects. It can simply append new objects at the end of the file, create a new xref table, and add a new trailer indicating that this new structure is now the active version.
This mechanism makes modifications fast, efficient, and safe, while also preserving all previous versions of the content.
How an incremental update works in practice
When a PDF is modified using editing software, it creates new objects corresponding to the modified elements. These new objects are appended to the end of the file without deleting the previous ones.
The software then creates a new xref table that references these new objects. A new trailer is added to indicate to the PDF reader where this new table is located. Finally, the startxref pointer, located at the very end of the file, is updated to point to this new structure.
When the file is opened, the PDF reader uses this pointer to access the latest version of the objects. Older versions remain present in the file but are ignored during display.
This means that a single PDF file can contain multiple successive versions of the same content, all stored within the file.
Why incremental updates are used in legitimate PDF software
Incremental updates were designed to meet legitimate and essential needs. They make it possible, in particular, to add electronic signatures without modifying the original document, which is crucial to ensure the integrity of digital signatures.
They also allow annotations to be added, forms to be filled out, comments to be inserted, or minor modifications to be made without risking file corruption. This mechanism improves the reliability of the PDF format and preserves the document’s technical history.
In many cases, the presence of incremental updates is therefore completely normal and does not constitute an indicator of fraud.
How fraudsters exploit incremental updates
The incremental update mechanism is frequently used in PDF document falsification. A fraudster can open an authentic document, modify sensitive information, and then save the file. The software then appends a new version of the modified objects, visually replacing the original information.
For example, a fraudster may modify the amount on an invoice, the salary on a payslip, a date, a name, or any other critical information. The displayed document appears perfectly consistent and authentic, because the PDF reader only displays the most recent version of the objects.
However, the original data remains present within the file. Forensic analysis makes it possible to recover it and demonstrate that the document was modified after its initial creation.
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 ?
Why these modifications are invisible to the naked eye
When a PDF is opened in a standard reader, it uses internal pointers to display only the most recent version of the objects. Older versions, although present in the file, are not accessible to the user.
The document therefore appears completely normal. There is usually no visual indicator that allows the modification to be detected. Even a careful inspection of the content cannot reveal the falsification.
This invisibility is precisely what makes incremental updates particularly dangerous in the context of document fraud.
How to detect incremental updates in a PDF
Detecting incremental updates relies on analyzing the internal structure of the PDF file. A document that has undergone incremental modifications typically contains multiple xref tables and multiple trailers.
Forensic analysis also makes it possible to identify the different versions of objects and determine when modifications were added. The presence of multiple startxref pointers is a clear indicator that the document was modified after its initial creation.
This analysis requires specialized tools capable of examining the internal structure of the file.
How to identify a suspicious modification
The presence of incremental updates is not, in itself, proof of fraud. Many legitimate documents contain incremental updates. However, certain characteristics may indicate a suspicious modification.
For example, a modification affecting an amount, a date, or an identity may be considered sensitive. A modification performed after the official issuance of the document may also be suspicious. Forensic analysis makes it possible to determine exactly which objects were modified and when.
This analysis makes it possible to establish whether the document has been fraudulently altered.
Typical example of fraud using incremental updates
A fraudster may download an authentic payslip in PDF format, open the file in a PDF editor, modify the salary amount, and then save the document.
The resulting file appears perfectly authentic. However, forensic analysis reveals that the document contains multiple versions of the same object. The original salary value is still present in the file, along with the modified version.
This discovery makes it possible to demonstrate that the document was falsified.
Why complete forensic analysis of a PDF is essential
Visual inspection of a PDF is insufficient to guarantee its authenticity. Even metadata analysis can be bypassed or manipulated.
Only a complete analysis of the file’s internal structure can detect incremental updates and identify invisible modifications. This analysis is an essential step in document verification processes, particularly in KYC, banking, real estate, or administrative contexts.
How TrustDocHub detects incremental updates and PDF falsifications
TrustDocHub analyzes the complete internal structure of PDF files in order to identify incremental updates and suspicious modifications. The system examines xref tables, trailers, objects, and internal pointers to reconstruct the document’s technical history.
This analysis makes it possible to detect invisible modifications, identify altered data, and assess the document’s risk level. This forensic approach ensures a high level of reliability in detecting document fraud.
Conclusion: incremental updates, a normal mechanism that can be exploited for fraud
Incremental updates are a fundamental mechanism of the PDF format. They allow fast and safe modifications, but they can also be exploited to falsify documents invisibly.
Detecting these modifications requires in-depth forensic analysis of the file’s internal structure. In a context where document fraud is increasingly common, this analysis is essential to ensure the authenticity of digital documents.
Understanding how incremental updates work is a key step in detecting falsifications and securing document verification processes.
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 ?
