We could use:
https://github.com/mozilla/pdf.js/blob/c3d08580627471432a6ad0b2fdb948df64126afa/src/display/api.js#L929

and look at bit 4.

The following pdf:
https://orimi.com/pdf-test.pdf
is not editable in neither Acrobat nor Edge nor Foxit and nor Okular.
value: typeof PDFJSDev !== "undefined" && PDFJSDev.test("CHROME") ? 2 : 0,
    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
  },
  viewOnLoad: {
    
    value: 0,
    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
  },
  cMapPacked: {
    
    value: true,
    kind: OptionKind.API,
  },
  cMapUrl: {
    
    value:
      typeof PDFJSDev === "undefined" || !PDFJSDev.test("PRODUCTION")
        ? "../external/bcmaps/"
        : "../web/cmaps/",
    kind: OptionKind.API,
  },
  disableAutoFetch: {
    
    value: false,
    kind: OptionKind.API + OptionKind.PREFERENCE,
  },
  disableFontFace: {
    
    value: false,