It takes a while for Virtual Analyzer to analyze PDF files.
When you check the usandbox.log, you see the following error message:
2014-01-09 14:39:57,731 [04076:04076][ ERROR] Curl error curl: (6) name lookup timed out
The issue happens because the customer's DDI management port doesn't have Internet access. This causes U-Sandbox to encounter timeout issues during in-the-cloud queries.
PDF files take more time to analyze because Sandbox tries to open PDF files using different versions of Adobe reader. However in this specific case, the issue of the extended analysis time was not as significant compared to the Internet access issue.
The issue can be resolved by providing Internet access to DDI's management port. This will improve new malware and C&C phone-home communication detection rate of DDI as wells as reduce the false positive rate.