Really need to use EphyDownload/WebKitDownload for PDF viewer (various PDF downloads broken)
We wound up using SoupSession rather than WebKitDownload to implement PDF.js, due to some timing bug in WebKitDownload, but turns out this was the wrong choice. By using a custom SoupSession, we lose the context of the browsing session, including login cookies, credentials, etc.
End result: I cannot download my phone bill. It should work if we figure out how to make WebKitDownload do what we want.