Learn Portuguese (Portugal): Envio-lhe o anexo.

In European Portuguese, object pronouns usually attach to the end of the verb with a hyphen when the verb is the very first word in the sentence. Because this statement starts with the verb envio, the indirect object pronoun lhe must follow it.

The translation says 'I send him', but could lhe mean someone else?

Yes! The pronoun lhe is used for the third-person singular indirect object. Depending on the context of your conversation, it can mean 'to him', 'to her', or 'to you' (if you are speaking formally to someone as você).

Why is there no word for 'I' in this sentence?

The form of the verb envio already tells us exactly who is doing the action. The ending -o is the standard 'I' ending for regular present tense verbs. In Portuguese, you can safely drop the pronoun eu (I) unless you really need to emphasize who is sending the attachment.

Is anexo the normal word to use for an email attachment?

Yes, anexo is the standard word used in Portugal for digital attachments in emails or messages.