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This is an old API-parameter that is now unused within the PDF.js project itself, and its description says that it's (partly) being used for "range requests operations".
Note that the `length` API-parameter is used to set the *initial* `contentLength` in various `BasePDFStreamReader` implementations, however it's always overridden by the "Content-Length" header (sent by the server) when that one exists *and* is a valid number. While we currently fallback to the keep the initial `contentLength` otherwise, note however how in that case range requests will always be *disabled* and thus the only spot in the code-base [where `fullReader.contentLength` is necessary](873378b718/src/core/worker.js (L230-L236)) cannot actually be reached.
Hence the only possible reason to use the `length` API-parameter would be for improved progress reporting[1] during streaming of PDF data in rare cases where the "Content-Length" header is missing/invalid, but the user *somehow* has information from another source about the correct `length` of the PDF document.
That situation feels very much like an edge-case, but it's obviously impossible to know if someone is depending on it. However, please note that there's a work-around available for users affected by this removal:
- Implement a `PDFDataRangeTransport` instance together with custom data-fetching[2], since in that case its `length`-parameter will always be used as-is.
Finally, updates various `BasePDFStreamReader` implementations to only set the `_isRangeSupported` field once the headers are available (since previously we'd just overwrite the "initial" value anyway).
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[1] I.e. to avoid the "indeterminate" loadingBar being displayed in the viewer.
[2] This is what e.g. the Firefox PDF Viewer uses.