I'm not sure if this has been mentioned but for hiding text documents, this may be handy. How To Hide Files in JPEG Images
Keep in mind that the file is only hidden for the naked eye, any form of deep packet inspection will not be fooled by this because the rar file is still identified by the Rar! header. Do not rely on this method to secretly transfer information.
There are a lot of ways to hide information in plain sight. Do not use words that will trigger the search-bots, anywhere in the file. Mix them up. For example, spell real the message out using every third letter in every word in a larger message. Or make the message into a picture, and then blur it like a capatcha. Put it in a video, in the text that streams as the video plays. Many ways. Riddles are the best, and the best riddles are the ones people don't even realize are riddles, except the intended recipients of the message. To everyone else it is something else. To the person that knows, there is a double meaning in what is presented and only they get it. Memes, that is a good way to hide information. Inside jokes. Etc.
