Here's the full trick:
Step 1: You will need two files - the file you want to hide and one jpg image - it can be of any size or dimensions. [If you want to hide multiple files in one jpeg image, just zip them into one file]
Step 2: Copy the above two files to the C: folder and open the command prompt window.
Step 3: Move to the c: root by typing cd \ [if the files are in another folder, you'll have to change the prompt to that folder]
Step 4: The most important step - type the following command:
copy /b myimage.jpg + filetohide.pdf my_new_image.jpg
To recover the original PDF file, just rename my_new_image.jpg to filename.pdf.
To restore the original file, rename the .jpg file to .rar and extract it using 7-zip or Winrar.
That's it - No advanced Steganography tricks involved here. Just remember to write the correct syntax of the copy command:
copy /b sourceimage.jpg + filetohide.ext targetimage.jpg