By some error invoices now read "Invoice no.Invoice 1241" (1241 is example only). I cannot delete the second word "Invoice". Invoices are sent by pdf. Help please.

Hi there, @ian-miller.

Welcome to the Community. I'm here to show you how to delete the information in QuickBooks.

The details that will show in the subject line will depend on your set up. I recommend going to the Account and settings section to correct the message.

Here's how:

1. Click the Gear icon in the upper-right corner.
2. Select Account and settings.
3. Go to Sales.
4. Under Messages, edit the information (ex. "Invoice no. [Invoice No.]") below Email subject line. 
5. Press Save. 
6. Hit Done.
7. Send an invoice again. 

For additional reference, I've attached a link you can use about sending invoices via email, as well as how to customise a sales form: Intuit Email and customise sales form.

Please let me know if you need anything else. The Community and I will be here to help you.