Can we password protect the generated Invoice or Packing Slips?

Yes this is possible, but not out of the box. You will have to add that to the $dompdf object in class-wcpdf-export.php, probably just before the render() call in line 130.
see: https://stackoverflow.com/questions/12190294/prevent-copying-content-in-a-pdf-that-is-created-with-dompdf for more information.

This is beyond the scope of this plugin (and support) though!

Hey @ewout i’ve tried your method, and the PDF gets password protected but it displays a blank pdf without any billing info.
Here’s the code! (extracted from plugin)

/**
		 * Create & render DOMPDF object
		 */
		public function generate_pdf( $template_type, $order_ids )	{
			$paper_size = apply_filters( 'wpo_wcpdf_paper_format', $this->template_settings['paper_size'], $template_type );
			$paper_orientation = apply_filters( 'wpo_wcpdf_paper_orientation', 'portrait', $template_type);

			do_action( 'wpo_wcpdf_before_pdf', $template_type );
			if ( !class_exists('DOMPDF') ) {
				// extra check to avoid clashes with other plugins using DOMPDF
				// This could have unwanted side-effects when the version that's already
				// loaded is different, and it could also miss fonts etc, but it's better
				// than not checking...
				require_once( WooCommerce_PDF_Invoices::$plugin_path . "lib/dompdf/dompdf_config.inc.php" );
			}

			$dompdf = new DOMPDF();
			$dompdf->load_html($this->process_template( $template_type, $order_ids ));
			$dompdf->render();
			do_action( 'wpo_wcpdf_after_pdf', $template_type );

			// Set Password
			$dompdf->get_canvas()->get_cpdf()->setEncryption('userpass', 'ownerpass', array('print'));

			// Try to clean up a bit of memory

			unset($complete_pdf);

			return $dompdf;
		}

		/**
		 * Stream PDF
		 */
		public function stream_pdf( $template_type, $order_ids, $filename ) {

			$dompdf = $this->generate_pdf( $template_type, $order_ids );
			$dompdf->stream($filename);

I know i must initiate the set password function between render and stream calls but the results i get is a blank password protected pdf with only black lines (from template i guess).

And if you get to develop this into a next version of the plugin, this will be the only plugin with such capability. (believe me i’ve searched everywhere)

Hi,
I think you should add the password before the render() call, rather than after it! Let me know if that helps!

Ewout