1. Save the scanned document in *.rtf or *.txt format
  2. In the gift record, go to File - Load file from...
  3. Browse to the saved filed
  4. Highlight the file and click Open
  5. Save and close the Notepad
  6. Save and close the gift

If you must use a different type of document such as PDF:
  1. Add the document to the media tab
  2. Include details in the Description field of the Media item to indicate which gift it is related to.
    Example - "This report pertains to Gift ID 1234 on 1/1/2014 for $1,000."