Create a directory in the source tree named "doc-files" in the package directory of the class, then put file.html in that directory. Then create a link to that file from the class. When you run javadoc, the file will be copied to the -d directory and the link will work.
Look at the example here:
http://java.sun.com/j2se/1.5.0/docs/tooldocs/windows/javadoc.html#unprocessed
-Doug