The word ‘annex’ in English is generally used for the smaller building attached to the side of a house. At university, there is sometimes an annex attached to the larger dormitory, where other students live. We do not use the word for something connected to an email.
We use the word attachment. If we have included a photo, a document or a pdf in an email, it is ‚attached‘ to the email. So, we would either say, ‘I have attached the document’ or ‘I have sent you an attachment.’
Another important distinction is between emails and letters. If we add something to an email, we attach it, but if we put something in an envelope, we ‘enclose’ it.
So, for example, we might say:
I have attached my CV since I forgot to enclose it in the original letter I posted.