Back to Knowledge Base
PDF Technical Spec

The XREF Table.

Understanding the internal pointer system that keeps your documents functional.

Definition

In the ISO 32000-1 specification (the PDF standard), the XREF (Cross-Reference) table is the "map" of the entire document. It contains the byte offsets for every object within the file. Without a valid XREF table, a PDF reader doesn't know where to find the text, images, or structure of the document.

Anatomy of an XREF Entry

A standard XREF entry consists of 20 bytes. It typically looks like this in the raw file buffer:

xref
0 1
0000000000 65535 f
14 5
0000000014 00000 n
0000000088 00000 n
  • Offset: 10-digit number representing the byte position from the start of the file.
  • Generation Number: Usually 00000, used for incremental updates.
  • Keywords: n for "in use" and f for "free".

Why PDFs Fail: XREF Corruption

When a PDF download is interrupted or a software crash occurs during saving, the byte offsets in the XREF table may no longer match the actual position of the objects. This results in the common "File is corrupted or damaged" error message.

Critical Knowledge

DocuStitch's Repair PDF tool works by scanning the entire binary stream of your file, identifying object headers manually, and reconstructing a perfect XREF table from scratch in your browser's RAM.

Modern PDFs: XREF Streams

In later versions of the PDF standard (1.5+), the table was often replaced by an XREF Stream. This allows the cross-reference data to be compressed, reducing the overall file size but making it more difficult to edit by hand.