In the earlier link "Graphics exported from Mathematica 9 are very large because even standard fonts are embedded" was pointed out and discussed. Indeed, I met the same problem when I used Mathematica 9.0.0.
So I change the version from 9.0.0 to 10.2.0. Things becomes worse since the PDF file which was 5MB generated by Mathematica 9.0.0, is now 28 MB.
Can anybody give me a help? Thanks a lot.