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.
outlineOptionsContainer: document.getElementById(
        "outlineOptionsContainer"
      ),
      currentOutlineItemButton: document.getElementById("currentOutlineItem"),
    },
    sidebarResizer: {
      outerContainer: document.getElementById("outerContainer"),
      resizer: document.getElementById("sidebarResizer"),
    },
    findBar: {
      bar: document.getElementById("findbar"),
      toggleButton: document.getElementById("viewFind"),
      findField: document.getElementById("findInput"),
      highlightAllCheckbox: document.getElementById("findHighlightAll"),
      caseSensitiveCheckbox: document.getElementById("findMatchCase"),
      matchDiacriticsCheckbox: document.getElementById("findMatchDiacritics"),
      entireWordCheckbox: document.getElementById("findEntireWord"),
      findMsg: document.getElementById("findMsg"),
      findResultsCount: document.getElementById("findResultsCount"),
      findPreviousButton: document.getElementById("findPrevious"),
      findNextButton: document.getElementById("findNext"),
    },
    passwordOverlay: {
      dialog: document.getElementById("passwordDialog"),
      label: document.getElementById("passwordText"),
      input: document.getElementById("password"),
      submitButton: document.getElementById("passwordSubmit"),
      cancelButton: document.getElementById("passwordCancel"),
    },