I'll guide you through the process of attaching documents to your sales transactions. To attach files on invoices, use the Attachment feature in QuickBooks Online. Here's how to attach files in the Attachments page:
Go to the Gear icon and select Attachments.
Click the Attachments icon to open the files.
Hold the CTRL button on your keyboard and select the items to upload.
Once everything is selected, release the CTRL button and hit Open.