The .webarchive file type used to save and view entire websites using Safari.
These files are typically created where a web page needs to saved and shared with others.
To open a .webarchive you will need to have Safari installed.
Safari for PC can be found on the Apple website, but note that it is an old version of Safari and Apple no longer provide updates.
Download Safari 5.1.7 for PC here: www.support.apple.com/kb/dl1531