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The proprietary nature of the software doesn’t matter. All that matters is that their FILES can be read and written. And with Office XML, since 2007, this is a solved problem.

They can export in an open format for publication, probably pdf. It shouldn't matter what's the format for edition.

The software is proprietary, but the actual files use a variant of an open ISO/IEC/ECMA standard.[1]

[1] https://en.wikipedia.org/wiki/Office_Open_XML

IIRC, it is essentially impossible to correctly implement parts of OOXML without reverse-engineering the Office suite. The standard does not provide enough information.