### Generate PDF Invoice from Order Data

Source: https://unidoc.io

This function generates a PDF invoice based on order details using a template. It requires the `creator` package from UniDoc and an `Order` struct. The output is written to an `io.Writer`.

```go
func GenerateInvoice(order Order, w io.Writer) error {
  c := creator.New()
  c.SetPageSize(creator.PageSizeLetter)

  tpl, _ := c.NewTemplate("invoice.tpl")
  tpl.SetContext(map[string]any{
    "order_id": order.ID,
    "total":    order.Total,
    "items":    order.Items,
  })

  return c.Write(w)
}
```

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