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.
pdfThumbnailViewer: null,
  
  pdfRenderingQueue: null,
  
  pdfPresentationMode: null,
  
  pdfDocumentProperties: null,
  
  pdfLinkService: null,
  
  pdfHistory: null,
  
  pdfSidebar: null,
  
  pdfSidebarResizer: null,
  
  pdfOutlineViewer: null,
  
  pdfAttachmentViewer: null,
  
  pdfLayerViewer: null,
  
  pdfCursorTools: null,
  
  pdfScriptingManager: null,
  
  store: null,
  
  downloadManager: null,