How do you decrypt a file with MacBook Air

You need to ask whoever encrypted that file. That's the purpose of encryption – to keep the file secure from unauthorized use. The idea is to limit access to specific people authorized to open, view, or otherwise use the file.

Sometimes, an encrypted file is sent via email, and the encryption key is sent via some other means; a text message for example, even a phone call assuming the key isn't particularly complex.

That's a broad question, but the general answer is that you need the file's encryption key.

The creator of the encrypted file would have to send the key to you.