I've got good news for you. You can add your instruction to the item description itself. This way, you won't have to paste it every time. I'll help you with the steps.
- Click Gear.
- Click Products and Services.
- Edit the item, then add the instruction in the Description field.
- Click Save and close.
QuickBooks won't automatically add a .pdf to the product. However, you can attach .pdf files when creating a transaction. You can add multiple file types as long as you won't reach the 25 MB file limit. Just make sure to put a check mark on the Attach to email box.
Feel free to refer to the link provided by @Xander above for more information about attachments.
You can also check this article in case you need a future reference: Income and expenses for QuickBooks Online.
Stay in touch if you have additional questions. We're always here for you.