Hybrid reference files are explained in ISO 32000-1 in section 7.5.8.4 Compatibility with Applications That Do Not Support Compressed Reference Streams.
it is possible to construct a file called a hybrid-reference file that is readable by readers designed only to support versions of PDF before PDF 1.5. Such a file contains objects referenced by standard crossreference tables in addition to objects in object streams that are referenced by cross-reference streams.
PS: It is not allowed, though, to freely mix both styles. As Leonard Rosenthol (Adobe PDF Architect & Principal Scientist and member of relevant standardisation committees) puts it,
you can NOT "cross the streams" (to quote the classic movie phase).
If the original PDF uses classic xrefs, you need to use the same at append time. If it uses streams, you need to use streams.
(Adding XRef table to PDF w/ XRef streams? on the Adobe PDF Language and Specifications forum)
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