The creation of the PDF/A standard was for the purpose of ensuring long-term, reliable archiving of digital documents. PDF/A-3 introduced a powerful new capability: the ability to embed arbitrary file types, such as XML, JSON, CSV, images, or even the original word processing file. This feature allows for the creation of hybrid documents that combine human-readable content with machine-readable data.

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In this blog post, we will explore the differences between PDF/UA and PDF/A-3a, helping you choose the right format for your business needs. We will discuss the key features, benefits, and use cases of each format to guide your decision-making process.

PDF/UA vs. PDF/A-3a: Which Format Should You Use for Your Business Application?

However, this flexibility comes with a strict requirement: every embedded file must include a correctly defined MIME type. This small piece of metadata plays a critical role in compliance, interoperability, and automated processing.

In this article, we explain why defining the MIME type of attachments in PDF/A is essential, not just a best practice.

PDF/A requires self-describing attachments

The primary purpose of PDF/A is long-term preservation. Documents should remain readable and interpretable decades from now, even if the original creators or software no longer exist. For attachments, this means the PDF must explicitly describe the type of embedded file.

By defining a valid MIME type (MIMEType), such as application/xml or text/csv, archivists and future software systems can reliably interpret the attachment. Without this information, the archival value of the embedded file is significantly reduced. The AFRelationship property (Relationship) defines the semantic relationship between a PDF document and its attachments. This relationship is essential for automated processing, accessibility, and compliance.

According to the PDF/A-3 specification (ISO 19005-3), both fields must be present and valid. Without these fields, the descriptive context of the attachment is incomplete, and the document is considered noncompliant. Validators such as veraPDF will mark documents without proper MIME typing as non-compliant.

Why the AFRelationship property is important

The AFRelationship property defines the semantic relationship between a PDF document and its attachments. This relationship is essential for automated processing, accessibility, and compliance. Common values include:

AFRelationship	Description
Source	The original source document from which the PDF was created, such as a Word or Excel file.
Alternative	An alternative representation of the main document content, such as a text version of a scanned image.
Data	Machine-readable data files, such as XML or JSON, that provide structured information related to the document.
Supplement	Additional information that complements the main document, such as appendices or reference materials.
Unspecified	Used when the relationship does not fit into predefined categories.

Defining the AFRelationship helps software programs and archivists understand how to handle the attachment. For instance, accounting software can automatically extract and process an XML invoice embedded as "Data," while a Word document marked as "Source" indicates that it is the original file used to create the PDF. Basically, it says why the attachment is there.

Interoperability across systems

Modern PDF/A workflows often involve multiple systems, such as scanners, enterprise resource planning (ERP) systems, accounting platforms, archive systems, and AI-based data extraction tools. These systems use MIME types to correctly process attachments. For instance, when an ERP system receives a PDF/A-3 document with an embedded XML invoice, it relies on the MIME type to identify and extract the XML data for processing.

• A ZUGFeRD, XRechnung, or Factur-X invoice embeds structured invoice data in the form of an XML file. The receiving system identifies the file by its MIME type, application/xml, and routes it to the correct parser.
• An embedded CSV file containing transaction data is identified by the MIME type text/csv, allowing accounting software to import the data seamlessly.
• Images embedded in a PDF/A-3 document, such as product photos or scanned signatures, are identified by their respective MIME types (e.g., image/jpeg or image/png), ensuring they are displayed correctly.

Ensuring correct MIME typing is therefore essential for ensuring consistent interpretation across applications.

Embedding attachments with TX Text Control

Text Control's document processing libraries fully support the PDF/A-3 standard. This includes correctly defining MIME types for embedded attachments. Developers can easily add different file types to their PDF/A documents while ensuring compliance with the standard.

The following code snippet demonstrates how to embed an XML file with the correct MIME type using TX Text Control in a .NET application:

// Serialize the XRechnung object to JSON
string json = JsonSerializer.Serialize(xRechnung);

// Generate the ZUGFeRD XML
string xmlZugferd = xRechnung.CreateXML();

// Load metadata from the XML file
string metaData = File.ReadAllText("metadata.xml");

// Create an embedded file for the ZUGFeRD invoice
var zugferdInvoice = new TXTextControl.EmbeddedFile(
    "factur-x.xml",
    Encoding.UTF8.GetBytes(xmlZugferd),
    metaData)
{
    Description = "factur-x",
    Relationship = "Alternative",
    MIMEType = "application/xml",
    LastModificationDate = DateTime.Now
};

// Configure save settings with the embedded file
var saveSettings = new TXTextControl.SaveSettings
{
    EmbeddedFiles = new[] { zugferdInvoice }
};

// Create, modify, and save the PDF document
using (var tx = new TXTextControl.ServerTextControl())
{
    tx.Create();
    tx.Text = "Test Document";
    tx.Save("test.pdf", TXTextControl.StreamType.AdobePDFA, saveSettings);
}

Foundation for automation and AI workflows

As AI becomes a standard component of document workflows, the value of structured attachments increases. JSON and XML attachments can be used to feed LLMs, enrich document semantics, and serve as machine-readable ground truth.

However, automated workflows depend on predictable metadata. MIME types are often the key signal used for:

• Selecting the right parsing strategy
• Applying validation rules
• Triggering domain-specific workflows
• Identifying attachments intended for machine consumption

An attachment may be skipped entirely or misinterpreted due to a missing or incorrect MIME type, which can break downstream automation. Therefore, defining MIME types is crucial for ensuring reliable AI-driven document processing.

Reducing ambiguity and security risks

Ambiguity caused by undefined or inaccurate file types poses a serious risk to archives, enterprise systems, and automated processing pipelines. For example, a system may misinterpret binary content, leading to parsing errors, corrupted data, or broken automation workflows.

Security filters may also fail to detect potentially unsafe content if the true nature of a file is obscured by a generic or missing content type. Long-term archives may not know how to restore, render, or migrate attachments when software ecosystems change. Malware scanners and sandboxing systems often rely on MIME types for initial classification, so missing or incorrect values weaken this first line of defense. Compliance workflows may treat unclassified attachments as untrustworthy and block their automated ingestion into regulated systems.

PDF/A enforces unambiguous classification of every embedded file by requiring explicit MIME typing. This improves the reliability, security, and auditability of archived documents, ensuring that attachments can be safely processed, validated, and preserved over time.

Conclusion

Defining MIME types for PDF/A-3 attachments is not just a best practice. It's a fundamental requirement for compliance, interoperability, and reliable long-term archiving. Properly typed attachments ensure documents are self-describing, easily interpretable, and seamlessly integrated into modern automated workflows.

With TX Text Control, developers can create PDF/A-3 documents with confidence, knowing that they will meet these stringent requirements and that embedded files will be correctly identified and processed across diverse systems and applications. Adhering to these standards helps organizations safeguard the integrity and accessibility of their digital documents for years to come.