The CBEC vide circular no. 1038/26/2016-CX dated July 19, 2016 has clarified that a manufacturer or a service provider who opts to issue invoices authenticated by digital signature may print a copy of such invoice and sign them manually and forward the same to such customers who are unable to accept or receive the digitally signed invoices. Further, it was clarified that such invoices would be considered to be in conformity with rule 11 of Central Excise Rules, 2002 or rule 4A, 4B and 4C of the Service Tax Rules, 1994 and such invoices would also be a valid document to avail Cenvat Credit under Cenvat Credit Rules, 2004.
Read more at: http://www.cbec.gov.in/resources//htdocs-cbec/excise/cx-circulars/cx-circulars-2016/circ1038-2016cx.pdf