Before you can enjoy this feature, you must be using Whatsapp v2.12.493 or higher versions. Download it directly from playstpre or from the link below
Download Latest Whatsapp v2.12.493 or older versions Here
Sending a document files is very easy, Just tap the link button on the top right corner, click on Document and navigate to the Document File you want to share.
Note: The File limit is 16Mb, files higher than 16MB cannot be shared.
In the Next coming updates, I think Whatsapp Messenger will support apk file sharing. What you think?