In PR #13214 we implemented rendering digital signature annotations, but they are not validated yet. For mozilla-central we display the fallback bar with a clear warning that the signatures are not validated so the user is aware of this, but we have no such mechanism for the generic viewer yet.

I'm thinking we could implement something similar to the fallback bar, which might even be more generally useful for other kinds of messages we now display in the fallback bar. For mozilla-central we'd then use the current browser fallback bar, and for other targets we would display the PDF.js fallback bar instead.

Alternatively we could perhaps add a semi-transparent overlay over the signature.
rendering_error: "An error occurred while rendering the page.",
  page_scale_width: "Page Width",
  page_scale_fit: "Page Fit",
  page_scale_auto: "Automatic Zoom",
  page_scale_actual: "Actual Size",
  page_scale_percent: "{{scale}}%",
  loading: "Loading…",
  loading_error: "An error occurred while loading the PDF.",
  invalid_file_error: "Invalid or corrupted PDF file.",
  missing_file_error: "Missing PDF file.",
  unexpected_response_error: "Unexpected server response.",
  printing_not_supported:
    "Warning: Printing is not fully supported by this browser.",
  printing_not_ready: "Warning: The PDF is not fully loaded for printing.",
  web_fonts_disabled:
    "Web fonts are disabled: unable to use embedded PDF fonts.",
};
function getL10nFallback(key, args) {
  switch (key) {
    case "find_match_count":
      key = `find_match_count[${args.total === 1 ? "one" : "other"}]`;
      break;
    case "find_match_count_limit":
      key = `find_match_count_limit[${args.limit === 1 ? "one" : "other"}]`;
      break;
  }
  return DEFAULT_L10N_STRINGS[key] || "";
}
// Replaces {{arguments}} with their values.
function formatL10nValue(text, args) {