It sounds like there may be one of two problems.
The pdfserver may not be able to render the pdf
The Splunk server is not delivering the search result to the pdf server
For issue 1, I would double check the error to make sure it is truly a timeout issue and not a local firefox process issue. Does firefox have proper resources to render the pdf?
For issue 2, it is possible that the search is timing out internally. I would check to see how long the search takes to run and see if it is hitting some sort of limit internally. Also, sending a 10k event result might be hitting another limit depending on your search.
The following debugging page might be helpful:
http://www.splunk.com/wiki/Community:TroubleshootingPDFServer
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