Thanks for getting back to me, Joyce-Lyn.

You have me to help you send a credit note. Let me guide you with these steps:

1. From the + New button, select Adjustment note.
2. Select a Customer name.
3. Enter their Email.
4. Enter the PRODUCT/SERVICE, QTY, RATE, and AMOUNT.
5. Select their GST.
6. Scroll down and click the Attachment icon.
7. Select the file and then click Open.
8. When done, click Save snd send.

I've also added this article that has the lists of transactions that can keep attachments: Attachments in QuickBooks Online.

Feel free to get back to this thread if you need my help. Take care always.