In the formal bussiness letters, can we use the word " attach" in stead of enlcose ??
The word 'enclose' is used in physical letters because you put something else in the envelope. It wouldn't be wrong to say 'attach' if you stapled pages together - in this way one page is clearly attached to another. This meaning has been taken up by internet mailing language so we use 'attach' and 'attachment' in mailing programs.