Plist files have two standard encodings, an XML-based textual one and a binary one. You can use plutil to convert between the two, e.g.
plutil -convert xml1 com.apple.Finder.plist
However, for preferences you probably want to use the `defaults` command instead, because IIRC they’re managed by a daemon (cfprefsd) that might not notice if you modify the backing files while it’s running.