I've just downloaded and unzipped it without any issue.
a) Ensure you have downloaded it from the official source: Oracle SQL Developer Downloads
b) Ensure your WinRAR version is up to date
c) If WinRAR isn't working, try a different unzip utility (Windows can unzip archives itself anyway). I used the free 7Zip to do it.
d) If all else fails, try downloading it again in case the download became corrupted on the way