Exception "Format string is not in the correct format" is thrown when importing a file with the following format string: "US$"#,##0.0"m";($#,##0.0)

The exception is thrown in the ValidateDateTimeFormatDescriptor method. 

Add support for Linked Timestamp Correctness.

Invalid structure tree triggers NullReferenceException instead of InvalidStructureTreeException.

Concurrent PDF export in a fresh process may hang, throw exceptions, or produce incorrect font embedding.

Handle duplicate annotation references across pages during import.

RtfFormatProvider: Merged cells with preferred widths narrower than their full column spans lose their spans on export.