"Downloader" is a streaming feature of client only mode which allows it to efficiently fetch PDF files that have been linearized. The PDF spec describes linearization as a feature "that enables efficient incremental access of the file in a network environment."

By default downloader is enabled which allows WebViewer to quickly display the initial page in the document and download remaining pages incrementally for large linearized files.

Generally it is not recommended to disable downloader because of the improved performance when loading linearized documents. It is only advisable if your server doesn't support byte range requests and you aren't able to enable them for some reason.

You also need to disable it if you are using the following page operations: doc.insertPages or doc.insertBlankPages.

You can disable downloader as follows:

WebViewer({
  initialDoc: "GettingStarted.pdf",
  useDownloader: false
}, viewerElement);