Generate a barcode from a different attribute (eg. a UPC code already in the system)
1. Copy /app/design/frontend/base/default/template/fooman/pdfcustomiser/item.phtml to your own theme
2. In the copied file, change
$pdfItem['productDetails']['Sku'],
$this->getPdfHelper()->getBarcodeType(),
to
$pdfItem['productDetails']['custom'],
$this->getPdfHelper()->getBarcodeType(),
3. Select your UPC attribute from System > Configuration > Pdf-Printouts > Custom Column Attribute.
Note that if you are using multiple custom column attributes, you might have to change the bolded custom to custom2, custom3, custom4 or custom5.