Use watermark and stamp for authentication.

1. Use watermarks: Add watermarks to the copies, such as "Only for handling XX matters", to identify their use and restrict copying. The watermark should use special fonts, colors or patterns to increase copying. Difficulty.

2. Stamp certification: Stamp the copy for certification, which can be an official seal or a personal signature seal. This can ensure the authenticity and legality of the copy and prevent others from malicious tampering.