You might be missing the fonts on the server perhaps. The fonts that are defined in the style sheets must be installed on the server in the correct location for the DITA-OT to use them when your Chinese PDF is being generated. Typically these go into the Windows Fonts folder. 

Assuming the PDF is generation is based on XSL:FO, the font-mappings.xml config file defines what physical fonts get mapped to the logical fonts per locale.  The file is located at App\Utilities\DITA-OT\{OT Instance}\plugins\{plugin|\cfg\fo.  If you are using Antenna House for the PDF composition,  the default location for fonts is c:\windows\fonts, but that can be changed in the config file font-config.xml that resides in the root folder of the Antenna House program Installation directory.  Keep in mind that if your PDF OT plugin is based on / builds upon another PDF plugin ( like PDF2) , the font mappings may be located in the base plugin and not the one you are using.

FYI. The Chinese font issue in the PDF is now fixed. Some Chinese characters are no longer displayed as a square or rectangular box, or as a box with a dot, question mark or “x” inside, etc.

 Cause:

• Some fonts that previously were included in our old SDL LCA 11 have been manually installed but may have been moved into optional font packages, and so may not be detected in our new SDL TD14.
• Our default fallback CJK language – Japanese used in our old SDL LCA 11 is not working in the new SDL TD14.

 Issue has been fixed after:

• the Simplified & Traditional Chinese Supplemental fonts (Windows optional feature) has been installed in the CMS servers.
• the default CJK language in the AH settings has been changed from Japanese to zho-CH (Simplified Chinese).