The issue is related to the data files (poppler-data, a package for encoded data) which do not install by default. We can install it and make the PDF display works with:
# apt-get install poppler-data.
Now the characters shown correctly. If the characters are shown as unknown symbols, it may related to the font configurations. You can alter your font settings using Droid fonts with this reference: 套用 Droid 字型到 ubuntu linux. Enjoy it. 🙂