Transparency works correctly, but it should be enabled using plot settings when you need to export it into a .pdf file:
OdDbLayoutPtr pLayout = OdDbBlockTableRecordPtr(pDb->getActiveLayoutBTRId().safeOpenObject())->getLayoutId().safeOpenObject(OdDb::kForWrite);
pLayout->setPlotTransparency(true);
exportToPdf(pDb);
Please also remember that since version 4.2, the PDFExportParams class does not contain the kSupportTransparency flag any more.