What is Hybrid-reference PDF file?

A hybrid-reference PDF is a PDF 1.5 or later file that combines objects referenced by conventional cross-reference tables and objects in object streams referenced by cross-reference streams within a single revision.

Identified by the XRefStm entry in the trailer dictionary, it supports compatibility with legacy PDF software unable to read cross-reference streams (ISO 32000-2:2020, clause 7.5.8.4).

For example, page tree objects may use conventional references for display in older software, while logical structure objects in streams remain invisible to such applications. Not to be confused with PDFs using both reference types across different revisions.