How to use the @iiif/vocabulary.MediaType.PDF function in @iiif/vocabulary

this._extensionRegistry[ExternalResourceType.SOUND] = {
            load: this._extensions[Extension.MEDIAELEMENT]
        };

        this._extensionRegistry[RenderingFormat.PDF] = {
            load: this._extensions[Extension.PDF]
        };

        // presentation 3

        this._extensionRegistry[MediaType.JPG] = {
            load: this._extensions[Extension.OSD]
        };

        this._extensionRegistry[MediaType.PDF] = {
            load: this._extensions[Extension.PDF]
        };

        this._extensionRegistry[MediaType.AUDIO_MP4] = {
            load: this._extensions[Extension.AV]
        };

        this._extensionRegistry[MediaType.VIDEO_MP4] = {
            load: this._extensions[Extension.AV]
        };

        this._extensionRegistry[MediaType.WEBM] = {
            load: this._extensions[Extension.AV]
        };

        this._extensionRegistry[MediaType.THREEJS] = {