Update "viewer.php" to PDF.js 4.0.189

The "viewer.php" acts as the "viewer.html" file included in the PDF.js
package. This commit updates "viewer.php" with the changes to
"viewer.html" from PDF.js 3.11.174 to 4.0.189 while respecting the
customized parts of "viewer.php".

Signed-off-by: Daniel Calviño Sánchez <[email protected]>

Author: danxuliu