Hi Dao,
Thank you for your interest in Syncfusion products.
We are able to reproduce the issue in displaying the PDF document in the PDF Viewer control. We have analyzed the document provided and found that the document uses Type1 (CID) fonts. At present we do not support displaying the text that uses Type1 (CID) fonts in our PDF Viewer. We have logged a Feature report for the implementation of this feature. Please open a Direct-trac incident to follow up the implementation of this feature.
Please let us know if you have any concerns.
Thanks,
Suresh