Initialize PDFView.url = ''

The absence of the `PDFView.url` property caused the following error in `PDFView.download`:

> Cannot read property 'split' of undefined

This property is unset when the PDF was loaded through a typed array instead of a URL. With the fix, the file name will default to "document.pdf", as defined by `getPDFFileNameFromURL` in web/ui_utils.js