is there any letter template (formal or informal)editing module available?

where admin can generate a Letter Template then

user or admin can edit the letter template to change the address, date, content, signature etc

and then print it in either in pdf or web form.......

if this module is unavailable what current module is best for me to customize to meet my requirements?

i know it is more easy if we use offline word processing but my friend insisting me to do this

Not sure what you are meaning by letter template, but you may want to try something like Smartsection.