add_filter('frm_pdfs_export_file_name', 'prepend_site_name' , 10, 2);
function prepend_site_name( $file_name, $args ) {
	$file_name = sanitize_title( get_bloginfo( 'name' ) ) . '-' . $file_name;
	return $file_name;
}

Use this code example to prepend the generated PDF file name with a value from the entry. (i.e. name of the user that created the entry)

add_filter('frm_pdfs_export_file_name', 'prepend_field_value' , 10, 2);
function prepend_field_value( $file_name, $args ) {
	$entry_id=$args['entry']->id;
	
	$fieldvalue=FrmProEntriesController::get_field_value_shortcode(array('field_id' => 2, 'entry' => $entry_id)); //change 2 to the ID of the field where the unique value is stored in your form
	
	$file_name = $fieldvalue. '-' . $file_name;
	return $file_name;
}

If you need to append the field value to the PDF filename, replace line 7 with the following:

$file_name = $file_name. '-' . $fieldvalue;

Use this code example to change all PDFs downloaded for the form to match their associated post title made from a Post action. Replace 23 with your form ID.

add_filter('frm_pdfs_export_file_name', 'use_post_title_for_pdf_filename' , 10, 2);
function use_post_title_for_pdf_filename( $file_name, $args ) {
	$target_form_id = 23; //Change 23 to the ID of the form.
	if ( $target_form_id !== (int) $args['entry']->form_id ) {
		return $file_name;
	}

	$post_id = $args['entry']->post_id;
	if ( ! $post_id ) {
		return $file_name;
	}

	$post = get_post( $post_id );
	if ( ! $post || ! $post->post_title ) {
		return $file_name;
	}

	return $post->post_title;
}