On the top row icons, there is an additional Attach File icon drawn as a paper clip. After you fill in your bills, click this icon to open "Attachments - Bills" dialog box:
In “Attach From:” field, select whether you want to attach your file(s) from your Computer, Scanner, or Doc Center.
Computer. Choosing this will open your computer to select your files from your computer.
Scanner. Selecting Scanner will open QuickBooks Scan Manager which automatically finds any scanner connected to your computer. Put your document in your scanner and click Scan.
Doc Center. If you had attached your files previously in bills or any other forms, your document would show in the Doc Center, otherwise it would show nothing as in above screenshot.
Once your document is attached, you can always look at your source document anytime right in your bills or other forms.