public function background($file)
    {
        $this->constrainSingleFile();
        $this->getCommand()
            ->setOperation('background')
            ->setOperationArgument($file, true);
        return $this;
    }
public function background($file)
    {
        $this->constrainSingleFile();
        $this->getCommand()
            ->setOperation('background')
            ->setOperationArgument($file, true);
        return $this;
    }
public function background ( $ file ) { $ this -> constrainSingleFile ( ) ; $ this -> getCommand ( ) -> setOperation ( 'background' ) -> setOperationArgument ( $ file , true ) ; return $ this ; }
Apply a PDF as watermark to the background of a single PDF file.

The PDF file must have a transparent background for the watermark to be visible.

@param string $file name of the background PDF file. Only the first page is used.
@return Pdf the pdf instance for method chaining
Apply a PDF as watermark to the background of a single PDF file .