Don't get me wrong, it's great that @calixteman wanted to implement that — JS should definitely be supported! I don't want to come off as rude or attacking.

For starters, JS reference specified in the [commit comment](https://github.com/mozilla/pdf.js/commit/8de98079ca05f8f7001ee077771055a57dc0e948) is outdated. It's version 8.1 from 2007… We're more than decade past that now. We should be implementing [Acrobat DC SDK](https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/AcrobatDC_js_api_reference.pdf) ([here](https://help.adobe.com/en_US/acrobat/acrobat_dc_sdk/2015/HTMLHelp/#t=Acro12_MasterBook%2FJS_API_AcroJS%2FJavaScript_API.htm) in a nice HTML version).

Other than that, the Doc object now is only implemented with `info` property with standard fields (`Title`, `Author`, `Subject`, `Keywords`, `Creator`, `Producer`, `CreationDate`, `ModDate` and `Trapped`, case insensitive). But [according to the spec](https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/AcrobatDC_js_api_reference.pdf#page=243) (even [in that old version](https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/js_api_reference.pdf#page=243)), this object can contain other, non-standard properties, that can be set both in Acrobat interface and in JS (which are case-sensitive then).

Acrobat 9.0 introduced another field to that property, `Authors`, containing semicolon-delimited list of authors.

There are probably other problems with that code due to the outdated spec @calixteman was apparently working on.
let rdf = xmlDocument.documentElement;
    if (rdf.nodeName.toLowerCase() !== "rdf:rdf") {
      
      rdf = rdf.firstChild;
      while (rdf && rdf.nodeName.toLowerCase() !== "rdf:rdf") {
        rdf = rdf.nextSibling;
      }
    }
    const nodeName = rdf ? rdf.nodeName.toLowerCase() : null;
    if (!rdf || nodeName !== "rdf:rdf" || !rdf.hasChildNodes()) {
      return;
    }
    const children = rdf.childNodes;
    for (let i = 0, ii = children.length; i < ii; i++) {
      const desc = children[i];
      if (desc.nodeName.toLowerCase() !== "rdf:description") {
        continue;
      }
      for (let j = 0, jj = desc.childNodes.length; j < jj; j++) {
        if (desc.childNodes[j].nodeName.toLowerCase() !== "#text") {
          const entry = desc.childNodes[j];
          const name = entry.nodeName.toLowerCase();
          this._metadataMap.set(name, entry.textContent.trim());
        }
      }
    }
  }
  get(name) {