I will. Suppose you are A, someone B will claim to be you, but tamper with your public key PUa and publish it to C, and this person knows the private key PRb corresponding to this public key PUb.

When he signed the file with this private key PRb and sent it to C, C would authenticate with PUb, and the authentication was successful.

So c will think that it is indeed a file.

The solution to this problem is to have a trusted organization responsible for issuing public keys. Avoid counterfeiting incidents.

This organization becomes CA, certification authority, and related facilities become PKI (Public Key Infrastructure).