QFontEngineDirectWrite doesn't override QFontEngine::properties and therefore the PDF export code can't get the correct bounding box and other information when embedding fonts into PDF. This means that exported PDFs give warnings when opened in Acrobat Reader and look incorrect.
To see this, use the attached test app. Turn on the 'DirectWrite' checkbox and click the 'Save as PDF' button. This should save 'test.pdf' in the same folder as the test app itself. Open this PDF in Acrobat and observe that the glyphs are the wrong size. Also I get a warning about an invalid bbox in the embedded font from Acrobat.