You've joined the right thread, Jean. I'll gladly show you how to ensure attachments are sent and accessible
to your customers in their invoices.

After adding your file from the Add Attachment section when creating an invoice, you'll need to tick the Attach to email box to show the attachment when you send it to your customer email.

Also, to ensure the attachment is accessible and sent with the invoice, tick the Show full details in email option and place a checkmark on the PDF Attached box from the Online Delivery section.

Here's how:

1. Go to the Gear icon and select Account and Settings.
2. Select the Sales tab.
3. Scroll down and click Online Delivery.
4. Tick the Show full details in email and PDF Attached.
   
   
5. After that, click Save, then Done.

For more details about the different file types that you can attach to QuickBooks transactions, together with things you need to know when emailing them, you can read this article: What transactions can keep attachments?

I'll also share this article to help personalize your sales form: Customize invoices, estimates, and sales receipts in QuickBooks Online.

In case you need further assistance sending invoices with attachments inside the program, you can also post them here again. We'll make sure to get back to you right away. Take care, and have a nice day!