When sending an invoice, from quickbooks to a customer, the pdf attachment is no longer attached to the email , when the recipient receives it.

Hello there, @kenny13.

You may check the option to attach the .pdf file to the invoice using the Custom Form Styles feature. I'd be happy to show you how to do that:

1. Go to the Settings, select Custom form styles.
2. Locate the invoice template and click Edit under the Action column.
3. Select the Emails tab and check the box beside PDF Attached.
4. Select Done. This is how it looks like:

To know more about how to customize sales forms, you may check out this article: Customize invoices, estimates, and sales receipts in QuickBooks Online.

I've also added this article for your future task about creating multiple invoices from one job or transaction for your customer: Set up and send progress invoices in QuickBooks Online.

Please don't hesitate to tag my name in the comment section below if you have other questions regarding QuickBooks. I'd be glad to help you further. Take care and have a wonderful day ahead.