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.
documentProperties: {
      dialog: document.getElementById("documentPropertiesDialog"),
      closeButton: document.getElementById("documentPropertiesClose"),
      fields: {
        fileName: document.getElementById("fileNameField"),
        fileSize: document.getElementById("fileSizeField"),
        title: document.getElementById("titleField"),
        author: document.getElementById("authorField"),
        subject: document.getElementById("subjectField"),
        keywords: document.getElementById("keywordsField"),
        creationDate: document.getElementById("creationDateField"),
        modificationDate: document.getElementById("modificationDateField"),
        creator: document.getElementById("creatorField"),
        producer: document.getElementById("producerField"),
        version: document.getElementById("versionField"),
        pageCount: document.getElementById("pageCountField"),
        pageSize: document.getElementById("pageSizeField"),
        linearized: document.getElementById("linearizedField"),
      },
    },
    errorWrapper,
    printContainer: document.getElementById("printContainer"),
    openFileInput:
      typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")
        ? document.getElementById("fileInput")
        : null,
    debuggerScriptPath: "./debugger.js",
  };
}
function webViewerLoad() {