Tested in latest Chrome and FF:

If you have multiple PDF files embedded as Iframe (via viewer.html) and at least one of them is password protected, the browser automatically jumps to the (first) iframe that shows the password prompt.

Imho, the iframe shouldn't modify the behavior of it's parent in any way.

See https://support.softcreatr.com/thread/4360-pdf-test/ for a demo.
evt.preventDefault();
  evt.returnValue = "";
  return false;
}
/**
 * Converts API PageLayout values to the format used by `PDFViewer`.
 * NOTE: This is supported to the extent that the viewer implements the
 *       necessary Scroll/Spread modes (since SinglePage, TwoPageLeft,
 *       and TwoPageRight all suggests using non-continuous scrolling).
 * @param {string} mode - The API PageLayout value.
 * @returns {number} A value from {SpreadMode}.
 */
function apiPageLayoutToSpreadMode(layout) {