This is a left-over from very old code[1], before there were a lot of `getDocument` options and when most of the library configuration was done via the (since removed) `PDFJS` global.
Given all the functionality added through the years, which require configuration[2], in practice it's now unlikely that calling `getDocument` without additional options will work except for the most trivial PDFs.
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[1] If the `getDocument` function was added today, there's no chance that it'd support anything other than a parameter object.
[2] Note things such as CMaps, standard fonts, wasm-based image decoders, and ICC-based colour spaces.
