Hide Files Inside Files!!

I can say it simply as merging two files into another file, whithout making them unreadable.

You can use such files by opening it with corresponding softwares. For example, if you hide an Archive file in a PDF file, then you can Access Files inside that using WinRAR, or Read It it using PDF Reader

Here is an Example :
1. Create a rar file(archive). Add some files to it. keep it in a folder. Now bring a pdf file into that folder.
2. Now open Command Prompt and bring the pointer to that folder by cd command.(cd /d "folderpath")
3. Now type the command given below:
copy /b "file1.ext1"+"file2.ext2"
"output.ext"

By pressing Enter you have been created that special file.
To test it, Right Click on that file and select "Open With" and then select "WinRAR". You can see the files in it. Also you can open the same
file in PDF reader by double clicking on it and you can read that pdf file.

using this trick you can create so many types of files containing another file(s) in them.
For example: pdf in jpg(photo), songs in jpg.