If the statement is made in the text of your letter, it may be fine as is, "herewith attached as reminder". Without seeing that part in context I'm not sure if it would be better or worse to say "as a reminder".
ps. Loob you used to insert the sentence at the end of the letter??
I insert "as a reminder" (and repeate the same for 3 following sentences) 'cause I just want to explain that I am not sending any additional document...but only remind what is in their hands...
is that Wrong??