Michael,
Nothing "outdated" about it.
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Byte Range Request - with regards to PDF this is associated with the "how" a Fast Web View (Linearized) PDF and a Server configured to 'page serve' gets it done.
Picture a large (page count) PDF parked on a server (file or web). You've 'clicked the link' and the PDF is on its way to your local machine.
With a properly configure server and a Linearized PDF the first few pages will be promptly rendered - you do not have to wait for the entire file to be transfered to the local machine.
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A nice detailed discussion of Linearized PDF is in Annex G of ISO 32000-1.
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Be well...