PDF (Portable Document Format) is a popular file standard for exchanging and printing documents.
In order to view PDF's, you must have a reader installed, such as Adobe Acrobat Reader
This can be downloaded from:

If you cannot open PDF's from within Outlook Express,
then make sure that your current security settings are not blocking attachments:
1. On the Tools menu, click Options, and then click the Security tab.
2. Un-tick "Do not allow attachments to be saved or opened that could potentially be a virus"