Change the slug of pdf viewer for wordpress from pdfviewer to anything you prefer. Put the code below in your active theme's fuctions.php file

change-pdfviewer-slug.php

if( ! function_exists( 'themencode_change_pdfviewer_slug' ) ):

	add_filter( 'register_post_type_args', 'themencode_change_pdfviewer_slug', 10, 2 );

	function themencode_change_pdfviewer_slug( $args, $post_type ) {

		if ( 'pdfviewer' === $post_type ) {
			$args['rewrite']['slug'] = 'pdfs'; // replace pdfs with your preferred slug
		}

		return $args;
	}

endif;

Make sure to navigate to Settings > Permalinks Page once after adding this code for the change to be propagated completely