The http handler is one way to handle all .pdf requests and register the handler in web.config
Please take a look at the following article
for handler registration in different versions of IIS.
The handler will handle cases when any .pdf file is requested by user.
As an alternative if you are using the download list widget from sitefinity to display a list of pdfs add logic to the widget template for download list widget that will redirect the user upon clicking a link to view pdf, here
is a blog post that explains how to perform modifications to existing widgets or if this is not the case let me know how how are the pdf files displayed on the site, in a custom control or sitefinity download list widget.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items