According to https://www.gnupg.org/gph/en/manual/x110.html
To decrypt a message the option --decrypt is used. You need the private key to which the message was encrypted. Similar to the encryption process, the document to decrypt is input, and the decrypted result is output.
Bob has a file, he wants to keep the file secret. He wants only wants his friend Alice, John and Paul to be able to look at the file.
He encrypts the single file with each of his friends public keys.
His intention is, that whenever for example Alice needs to look at the file, she does not need to be with the others to access it.
An Attacker finds the encrypted file.
What i'm asking is:
Would the attacker need to find out each of the friends private keys in order to decrypt it? Or would the attacker only need to know one of the friends private keys?