Step 1: Backup the file you are going to edit.
Step 2: Download any Hex Editor freeware. I have used Hex Editor Neo.
Step 3: Install and run hex editor.
Step 4: Open the file in hex editor.
Step 5: As shown in above image enter your secret data at end of file from non Hex panel.
Step 6: Save the file and open the file normally to check.
Step 7: To view the secret data again open the file in hex editor or you can open it in any text editor if the file edited is not a text file i.e. doc, txt, rtf
Tip: Editing an image file would be more recommended than text and exe files.