Hi Jinjun,
From my experience, this is usually a problem of the PDF rendering software not the actual PDF.
In particular, the paths appear thick when zoomed out but fine when zoomed in. This should not have an effect on actually printing the PDF (but this might depend on the printer and printer driver).
From a quick test with your sample (thanks for providing one!), see exactly this behavior -- but a print-out looks very good.
If you need to optimize for viewing in PDF rendering software, then the HTML-based outputs are usually better.
Best regards,
Peter.