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You could try to progressively render the file, pausing when a required forward reference isn't within the data you have downloaded so far.

That seems to be what they do, in my experience.

engineers gonna engineer

Well, I imagine that they download the file in the background, and if the viewer seeks to a non-downloaded part of the file, then it just blocks with "Loading" until that part is downloaded.

"Linearized" PDFs have extra info up front to allow streaming and random access (via range requests).

Actually, PDF files can be optimized for streaming: http://acroeng.adobe.com/wp/?page_id=27

It's covered in Annex F of the PDF spec: https://wwwimages2.adobe.com/content/dam/Adobe/en/devnet/pdf...

If I remember correctly the PDF Spec says the files should be read backwards. The object table is at the end of the file.