regarding signature

Yes, signature is mandatory for invoices under GST. Signature may be in either of the following ways:

By hand, by the authorised person or 

By way of affixing his digital signature (as approved / issued under the Information Technology Act, 2000). 

Note: The following will not qualify as signature / as signed invoices under the GST laws: 

If the signature of the authorised person is scanned and affixed as a picture on the invoice 

If the document contains a declaration such as 'this is a computer generated invoice and does not need a signature' 

If signed by a person, other than an authorised signatory

Got ur point sir thanks for ur valuable reply