The old implementation in `PDFViewerApplication.download` means that if the `getDocument`-call hasn't yet downloaded the *entire* PDF document it will be re-downloaded. This seems generally undesirable since:
- In some (probably rare) cases a URL may not be valid an arbitrary number of times, which means that the download may fail.
- It will lead to wasted resources, since we'll end up fetching the same PDF document *twice* in that case (once via the `getDocument`-call and once to allow the user to save it).
Hence this patch suggests that we change this very old code to instead always call the `PDFDocumentProxy.getData` method, since that'll trigger immediate downloading of the remaining document via the existing `getDocument`-call.
Finally, the patch removes the `PDFViewerApplication.downloadComplete` property since it's now unused.
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