Hi glarysolutions, thanks for reaching out. Unfortunately, QuickBooks doesn't have an option to add 'Digital Signature' at the moment. We request you to please write us feedback within QuickBooks from 'Feedback' widget. To write the feedback, please login to QuickBooks, then click on 'Settings Gear' icon and then click on 'Feedback' option which appears under 'Profile' option
I am not sure yet whether this workaround is applicable on QBO India version. For the time being, you should explore it in the trial account.
1. Utilize the statarter plan of this connector for free.
https://www.integromat.com/?pc=quickbooks
2. Set the trigger to save your PDF invoice in GDrive and sign it digitally by Eversign/Zoho Sign
3. Create any invoice and sent to your own email address to run the trigger of the connector.
4. Send your signed invoice as attachment