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.
kind: OptionKind.API,
  },
  maxImageSize: {
    
    value: -1,
    kind: OptionKind.API,
  },
  pdfBug: {
    
    value: false,
    kind: OptionKind.API,
  },
  standardFontDataUrl: {
    
    value:
      typeof PDFJSDev === "undefined" || !PDFJSDev.test("PRODUCTION")
        ? "../external/standard_fonts/"
        : "../web/standard_fonts/",
    kind: OptionKind.API,
  },
  verbosity: {
    
    value: 1,
    kind: OptionKind.API,
  },
  workerPort: {
    
    value: null,
    kind: OptionKind.WORKER,
  },