bug#43318: Unable to open remote pdf directly with eww

[Stefan Kangas]

Steps to reproduce:

0. emacs -Q
1. M-x eww RET https://www.gnu.org/software/emacs/manual/pdf/emacs.pdf RET

Result: My external pdf reader (titled "Document Viewer", not sure what
it is, maybe some Gnome thing) opens with the message "Unable to open
file '/home/skangas/wip/emacs/-'".  It seems like it tried to open the
file "-" in the directory from which I run Emacs.

It would be useful if this would open the pdf doc-view-mode, or if the
pdf was downloaded to /tmp and opened using "Document Viewer" from there
or something.