I want to attach a W9 form to a sales invoice. Many companies require W9s

Hello there, Mbrown. I'll outline the steps to obtain your contractor's W-9 form and attach it to a sales invoice in QuickBooks Online (QBO).

If you haven't yet, you can download a copy of your contractor's W-9 form on the Contractors page. Here's how:

1. Go to Expenses and click Contractors.
2. Click your contractor's name. 
3. Go to the Documents tab and select Download on the W9 section.

Once done, refer to the steps below to attach it to a sales invoice.

1. Go to + New and select Invoices.
2. Enter your customer's name and fill in the necessary fields.
3. Scroll down until you see the Attachments section. You can drag and drop the file or click the icon to attach the W-9 form.
4. Click Save and Close.

I'll share this article for the full guide: Add or delete attachments in QuickBooks Online.

Furthermore, you can take advantage of the available tools in QBO, so you can customize your invoices according to your preferences. 

Please don't hesitate to reach out to the Community if you have other concerns about attaching documents to invoices. I'll be more than happy to support you. Keep safe.