Reader fails on object streams (/ObjStm): "object N not found in xref"

[ElephantsFoot]

Description

The PDF reader does not support compressed object streams (/ObjStm, i.e. type-2 cross-reference entries). Any PDF written with a cross-reference stream (/Type /XRef, PDF 1.5+) that stores objects inside object streams fails to read/merge with:

pdf: object <N> not found in xref

Because most modern PDF producers emit object streams by default, this affects a large share of real-world PDFs — not an edge case.

Steps to Reproduce

Take any valid PDF and ensure it uses object streams (most modern tools do this already; to force it):

qpdf --object-streams=generate plain.pdf objstm.pdf

# confirm it uses xref + object streams
grep -a -c '/Type[ ]*/XRef' objstm.pdf   # -> 1
grep -a -c '/ObjStm'         objstm.pdf   # -> 1 (or more)

Then:

data, _ := os.ReadFile("objstm.pdf")
_, err := gpdf.Merge([]gpdf.Source{{Data: data}})
// err: "pdf: source 0: pdf: object <N> not found in xref"

Expected Behavior

The reader should support compressed object streams per the PDF spec (ISO 32000, cross-reference streams §7.5.8 and object streams §7.5.7):

• In parseXRefEntries, handle type-2 entries: field[1] = object number of the containing /ObjStm, field[2] = index of the object within that stream.
• Add an object-stream reader: given an /ObjStm object, decode its stream, parse the N header pairs (object number / offset) up to /First, and resolve the requested object from the decompressed body. Caching the parsed /ObjStm avoids re-decoding for sibling objects.

With those two pieces, GetObject resolves type-2 objects and object-stream-based PDFs read and merge correctly.

Actual Behavior

Reading/merging the PDF fails with:

pdf: object <N> not found in xref

The object exists in the file inside a compressed object stream (type-2 xref entry), but the reader ignores type-2 entries so GetObject never finds it. The file is valid and opens fine in macOS Preview.

Go Version

go1.26.4 darwin/arm64

gpdf Version

v1.0.11

PDF Viewer (if relevant)

No response

[ElephantsFoot]

I'd like to work on this and open a PR with a fix, but it looks like the repo restricts pull request creation to collaborators. Could I be added as a collaborator so I can submit one? Happy to follow whatever branch and review conventions you prefer. Thanks!