Currently PDF created by "Download as PDF" are of inferior quality than those generated by the desktop app.

Most notably, vector artwork such as SVG logos, even simples ones with flat colors, are preserved in Word fpr Desktop, resulting in nicer looking and smaller PDFs.

Word online rasterizes all vector art and makes them low-res, blocky images, which consume more space. This makes for visually unpleasant, unfaithful documents, which take up more space.

PDF generation on Word online should preserve vector images in the output for better visual fidelity and smaller documents.