Currently I'm reading an article about ProtonMail here and I don't understand it.
Now let’s address ProtonMail’s weaknesses. One of the big issues is that it isn’t easy to know whether a message sent to another ProtonMail user is being encrypted to the recipient’s correct public key, which is stored on ProtonMail’s keyserver. For example, if Alice sends Bob a message encrypted to his public key, it’s harder for anyone else to read the message. But since ProtonMail distributes the encryption keys to users, it has the technical ability to give Alice its own keys in addition to Bob’s, thus encrypting the messages in a way that would allow it to eavesdrop.
What is meant under 'its own keys'? You encrypt messages using public keys only and they are, well, public. So where is the problem in fact that someone knows someone elses public key?
UPDATE After comments and answer I realized that confusion goes away if you reword the highlighted sentence like this:
But since ProtonMail distributes the encryption keys to users, it has the technical ability to give Alice freshly generated malicious keys instead of Bob’s, thus encrypting the messages in a way that would allow it to decrypt them on the server (using private key from freshly generated keys) even without knowing Bob's private key.