PDF/A and Embedded Attachments - Full Detailed Explanation

An embedded attachment is a file stored inside a PDF document. Examples include:

• XML files
• CSV files
• Spreadsheets
• CAD files
• Images
• Source documents
• Digital invoices
• Supporting documentation

In standard PDF, any file type can be embedded using the EmbeddedFiles name tree and file specification dictionaries.

The relationship between PDF/A and attachments depends on the PDF/A conformance level and version.

Why PDF/A Restricts Attachments

PDF/A is an archival standard designed for long-term preservation.

The primary goals are:

• Self-contained documents
• Predictable rendering
• No external dependencies
• Long-term accessibility

Attachments can create preservation problems because:

• Software required to open the attachment may become unavailable.
• The attachment may contain executable content.
• The attachment may not be preserved correctly.
• The attachment may not be related to the archival purpose.

For these reasons, earlier PDF/A versions heavily restrict attachments.

PDF/A Versions and Attachment Support

PDF/A Version	ISO Standard	Attachments Allowed
PDF/A-1	ISO 19005-1	No
PDF/A-2	ISO 19005-2	Yes (restricted)
PDF/A-3	ISO 19005-3	Yes
PDF/A-4	ISO 19005-4	Yes
PDF/A-4e	ISO 19005-4	Yes
PDF/A-4f	ISO 19005-4	Yes

PDF/A-1 and Attachments

PDF/A-1 is based on PDF 1.4.

Embedded files are prohibited.

If a document contains:

/Names
   /EmbeddedFiles

the file cannot be PDF/A-1 compliant.

PDF/A-2 and Attachments

PDF/A-2 permits embedding only PDF/A-compliant documents.

Example:

Main.pdf
 └── Attached:
      ArchivedInvoice.pdf

If:

ArchivedInvoice.pdf

is also PDF/A-compliant, validation succeeds.

If attached file is:

invoice.xlsx

validation fails.

PDF/A-3 and Attachments

PDF/A-3 was a major change.

Any file type may be attached.

Examples:

• XML
• CSV
• DOCX
• XLSX
• ZIP
• CAD
• Images
• Other PDFs

The PDF document remains the archival object.

The attachment does not need to be archivable itself.

PDF/A-4 Attachment Model

PDF/A-4 is based on ISO 32000-2 (PDF 2.0).

Attachment support becomes cleaner and more standardized.

Key features:

• Embedded files allowed
• Associated files framework
• Better metadata integration
• Improved relationship handling

PDF/A-4 relies heavily on Associated Files (AF).

Associated Files (AF)

PDF 2.0 introduced the /AF array.

Example:

Catalog
   /AF [
      fileSpec1
      fileSpec2
   ]

This explicitly declares files associated with the document.

Benefits:

• Clear relationship
• Better preservation
• Machine-readable associations
• Improved validation

PDF/A-4e

The "e" means: "Engineering"

Designed for technical documentation.

Typical attachments:

• CAD
• STEP
• IFC
• BIM
• 3D engineering data

Example:

Drawing.pdf
 └─ model.step

Valid.

PDF/A-4f

The "f" means:: "File Attachments"

This conformance level explicitly supports arbitrary embedded files.

Example:

Package.pdf
 ├─ model.step
 ├─ source.xml
 ├─ spreadsheet.xlsx
 └─ image.tif

All valid.

Required Attachment Components

A valid attachment typically contains:

1. Embedded File Stream

/Type /EmbeddedFile

Contains the actual file data.

2. File Specification Dictionary

Example:

<<
  /Type /Filespec
  /F (invoice.xml)
  /UF (invoice.xml)
  /EF <<
      /F embeddedFile
  >>
>>

3. MIME Type

Should identify file content.

Example:

/Subtype (application/xml)

Examples:

File Type	MIME Type
XML	application/xml
CSV	text/csv
TXT	text/plain
PDF	application/pdf
JPEG	image/jpeg
PNG	image/png
DOCX	application/vnd.openxmlformats-officedocument.wordprocessingml.document
XLSX	application/vnd.openxmlformats-officedocument.spreadsheetml.sheet

4. AFRelationship

Defines how the attachment relates to the PDF.

Examples:

Value	Meaning
Source	Source document
Data	Supporting data
Alternative	Alternate representation
Supplement	Supplemental information
Unspecified	Relationship unknown

Example:

/AFRelationship /Data

---

Typical PDF/A Attachment Structure

Simplified example:

Catalog
 ├─ Metadata
 ├─ Pages
 ├─ AF
 │   └─ File Specification
 │       ├─ AFRelationship
 │       ├─ Embedded File
 │       └─ MIME Type
 └─ EmbeddedFiles

---

ZUGFeRD and Factur-X Example

Electronic invoice standards such as:

• ZUGFeRD
• Factur-X

commonly use:

• PDF/A-3
• Embedded XML invoice

Structure:

PDF/A-3
 ├─ Human-readable invoice
 └─ XML invoice attachment

Relationship:

/AFRelationship /Alternative

This means:

• PDF = visual invoice
• XML = machine-readable invoice

Both represent the same invoice.

Best Practices

Use PDF/A-3 or PDF/A-4

For modern workflows involving attachments.

Set Correct MIME Types

Always identify content accurately.

Use AFRelationship

Never leave relationships ambiguous when a known relationship exists.

Add Metadata

Include:

• File name
• Description
• Creation date
• MIME type

Quick Summary

Feature	PDF/A-1	PDF/A-2	PDF/A-3	PDF/A-4	PDF/A-4e	PDF/A-4f
Embedded Files	No	Yes	Yes	Yes	Yes	Yes
Any File Type	No	Restricted	Yes	Yes	Yes	Yes
Associated Files (AF)	No	Limited	Supported	Full PDF 2.0 Support	Full	Full
AFRelationship	N/A	Recommended	Required for AF	Required	Required	Required
Typical Use	Archive PDF	Archive + Data	E-Invoicing	Modern Archiving	Engineering	Archive + Attachments

In practice, PDF/A-3 and PDF/A-4f are the most common standards when documents need to carry embedded XML, spreadsheets, CAD files, or other supporting data. PDF/A-1 should be avoided for attachment-based workflows because it completely prohibits embedded files.