How can I change Email Subject that will contain customer name, invoice number and "from #company name" eg: "Test invoice #1234 from test Company"  ...Please suggest

Use the userevent script and the N/email module to send the email when the Invoice is created.

email is going through standard way and using advanced pdf template.... we dont want to use script... is there any other way

you should be able to modify the subject in the template that is being used to generate the emails... lmk if you need assistance with that.

@Sean Murphy I am using advanced pdf template

Yup you should be able to modify the subject in that temple. OR in the campaign that is controlling the email execution.

actually the email is going in standard way during invoice creation

Documents --> Templates --> Email Templates. Customize the Standard Transaction Email Template.... You can then change the subject line in there... Save the new template and go back to the invoice form Edit that form and and choose the new Email Message Template there. that should do it I think.