In the E invoice already E sign coming by default in this digital signature not required, but in your sales invoice we can configure it for this follow below steps:
Step 1: Go to Administration.
Step 2: Select Configuration, then Features & Options.
Step 3: In the General Options, set Apply Digital Signatures to Invoice Document to Yes.
Step 4: Click on the Configure tab.
Step 5: Specify the digital signature software vendor, select the digital signature certificate(s), specify the password or PIN, and specify the search string.
Step 6: After configuring this, go to the Transaction option.
Step 7: Select the sales voucher you want to configure.
Step 8: Click on Modify and enter the required voucher number to open the voucher.
Step 9: Open the sales voucher and click on the Sales Invoice Print option.
Step 10: Choose Apply a digital signature (only PDF) and select Yes.
Step 11: Export the invoice in PDF format.
Step 12: Once you export the invoice in PDF, you will see the digital signature applied to your sales invoice.