Reference XObjects

Background

PDF 1.4 introduced Reference XObjects, but it has not been implemented in Acrobat in the intervening years. With the growing popularity of PDF/X-5 documents and interest in variable data printing (VDP) Acrobat decided to add support for Reference XObjects in 9.0. This is likely to benefit Original Equipment Manufacturers (OEMs) and print providers to provide customized workflows for their customers by executing VDP.
So far, there were no consumers for documents containing Reference XObjects (including absence of it in Acrobat) because there were no producers for the same. Acrobat finally breaks this chicken and egg situation that has been prevalent for a while. With Acrobat's support of such documents, more applications will be encouraged to produce them.

What the PDF Reference says (Section 4.9.3):

Reference XObjects (PDF 1.4) enable one PDF document to import content from another. The document in which the reference occurs is called the containing document; the one whose content is being imported is the target document. The reference XObject in the containing document is a form XObject containing the optional Ref entry in its form dictionary. This form XObject serves as a proxy that can be displayed or printed in place of the imported content. The proxy might consist of a low-resolution image of the imported content, a piece of descriptive text referring to it, a gray box to be displayed in its place, or any other similar placeholder. The presence of the Ref entry distinguishes reference XObjects from other types of form XObjects.

When the imported content replaces the proxy, it is transformed according to the proxy object’s transformation matrix and clipped to the boundaries of its bounding box, as specified by the Matrix and BBox entries in the proxy’s form dictionary The combination of the proxy object’s matrix and bounding box thus implicitly defines the bounding box of the imported page.

For a detailed description see: PDF Reference Manual

Using Reference XObjects in Acrobat 9

In Acrobat 9, reference XObjects are enabled by default for PDF/X-5 documents only. To enable it for all documents, select Always as the View Mode for Reference XObjects in Preferences --> Page Display. The location of the target documents can be specified in the box below that.

Since reference XObjects access external content, security becomes a concern and to ensure that the documents are trust-worthy, the location of the containing documents must be specified in the Enhanced Security locations. This can be found in Preferences --> Security (Enhanced). All files and folders accessing external content must be added as trusted locations regardless whether the checkbox, Enable Enhanced Security is checked or not.

With these settings in place, Acrobat will look for target documents in the specified locations and use the imported content. If it does not find a target document, or if there is an error in the pdf, or if the containing document is not trusted, the proxy will be displayed.

Special Notes:
The Ref dictionary contains:

F - File specification dictionary. This is according to the file specification given in PDF Reference 1.7. For non-english characters to be supported across all platforms, the file name must be contained in UF, the text string entry.
Page - Integer or text string. A page index or page label identifying the page of the target document containing the content to be imported.
ID - array. An optional entry specifying a file identifier for the file containing the target document.