Q: Sometimes when I try to open a file or e-mail I get the message: "The selected file has an extension (PDF) that is not recognized by Windows Media Player but the player may be able to play it.

PDF, Flash, and Java: the Most Dangerous File Types Just about everyone uses Adobe's PDF format to transmit important documents, and websites that don't use at least a little Java aren't so common.

Over the years, Adobe's PDF file type has become a universally accepted method for sharing digital documents. The format's cross-platform adoption means the documents can be viewed on almost any ...