[3.4.0]

Added

• Added MediaDataPartType.Mp4 to support MP4 video media parts in presentations and documents (#1866)
• Updated bundled Open XML schemas to the Q3 2025 Office release, enabling newer document and presentation features (#1963)

Changed

• Reduced JIT and AOT size and improved document load performance by removing the generic builder pattern from element metadata creation (#1842, #1843)
• Optimized FromChunkedBase64String to significantly reduce allocations and improve throughput when decoding chunked base64 content (≈2.4× faster, ≈70% less memory) (#1868)

Fixed

• Switched to XmlDOMTextWriter instead of XmlWriter.Create to correctly handle serialization of certain XML content (#1869, #1961)
• Added a clear error when attempting to open encrypted documents that the SDK cannot process (#1635, #1969)
• Improved exception messages when required package parts are missing by including the name of the missing part (#1971, #1974)

Documentation

• Clarified the README SDK description and simplified the note on 3.0.0 breaking changes (#1858)
• Documented that disposing a package with AutoSave set to false does not persist changes (#1873)
• Removed the PowerPoint modern comments sample, which now lives on learn.microsoft.com (#1861)

Thanks to the following for their contributions:

@QuocDatHoang
@Varorbc