Hi,
I am working on a non-fiction book using diagrams I have created in Inkscape 0.92.1 r15371.
When I export diagrams as PNG and import into MS word they look fine in situ. When I then export the word file as PDF the diagrams become mostly black rectangles.

One might think this is an MS word issue, however, if in Inkscape I do save as PNG and put that in the word doc and export as PDF it is fine except the resolution is unacceptable.

So there must be some difference in the PNG files created by Inkscape by "save as" and those created by export.

Does anyone know of this issue?

I suppose that Word's PDF-saving version can't deal with transparency in your pngs.

Use an opaque background rectangle (e.g. a white one) in your pictures to work around the problem.

Alternatively, you can go to file/document properties and change background color to have maximum opacity (255). The Cairo PNG save is not a good option for many reasons but if you are stuck with it there are ways to address the resolution problem