Thanks to the use of a cutting edge cryptographic algorithm. Breaking it would lead to breaking the Bitcoin digital currency.
No way to cheat:
Your proof will be stored in the "Blockchain", shared by millions computers around the world. It would be impossible to intercept it or tamper with it.
Decentralized trust authority:
Once your document is certified, you don't need to rely on PROOF.com. You can do it yourself with just the certificate.