Javadoc is a part of the standard JDK.
It is as simple as nothing else. Just go by this link:
Download "JDK 6 Update 13". Install it on your computer. In the "bin" directory of your JDK installation you will find javadoc.exe. That's it!
Javadoc itself is a program that automatically generates HTML Java API documentation by any Java sources. Sometimes people call "JavaDoc" that very HTML documentation generated with javadoc.exe (no matter from what).
What you mean saying "a file javadocs.zip" is unclear. If you are looking for the standard Java API documentation (that describes all the Java classes which constitute the JDK) that documentation can be equally downloaded from the same page:
It is found there under the title "Java SE 6 Documentation".