Autotest use watin:how to close the pop-up PDF file?

When I use WatiN to test a website, for example,navigate to 'http://www.opm.gov/hcaaf_resource_center/assets/Lead_tool3.pdf',it wil开发者_运维百科l prompt a PDF file.How can I close it?

There are several posts about dialog handlers. Recent ones. Suggest a search "dialog handlers" in the watin section