code: public void addViewerPreference(PdfName key, PdfObject value) {
      	useVp = true;
      	this.viewerPreferences.addViewerPreference(key, value);
      }
docs: Adds a viewer preference
  @param key a key for a viewer preference
  @param value the value for the viewer preference
  @see PdfViewerPreferences#addViewerPreference