Here is my views on this:
1. The first purpose of raising an Invoice is as an Evidence and to create a right on good/service. The Sale of goods act and the Indian Evidence Act asks for a signed invoice.
2. The next purpose of raising an invoice is for taxes. The Maharashtra VAT also asks for a signature of any responsible person ( See >> http://mahavat.gov.in/Mahavat/MyFold/RTI/FAQ/Tax%20Invoices%20for%20VAT%20VAT%20leaflet%20105.pdf )
Here is a short-cut:
You can choose to mention in your first agreement with the client that
- Invoices will be sent from a particular email id,
- will be computer generated and
- will bear no signature and
- will still be binding.
If done so, you can do away with signing every time and send pdf invoices backed by this agreement.