Using the EWW browser, I can navigate around to webpages, but navigating
to a pdf fails. It appears to download an unreadable pdf. Downloading
and viewing the pdf manually in emacs (pdf-view-mode) works fine.

I've tried these solutions to no avail:
- Adding "application/pdf; emacsclient %s" to a ~/.mailcap file. This
  makes the process hang indefinitely.
- Running the following elisp code:
  `(add-to-list 'mailcap-user-mime-data
                 '((type . "application/pdf")
                   (viewer . pdf-view-mode)))`
  This doesn't seem to have an effect.


--System Info--
In GNU Emacs 27.1 (build 1, x86_64-w64-mingw32)
 of 2020-08-21 built on CIRROCUMULUS
Repository revision: 86d8d76aa36037184db0b2897c434cdaab1a9ae8
Repository branch: HEAD
Windowing system distributor 'Microsoft Corp.', version 10.0.19041
System Description: Microsoft Windows 10 Home (v10.0.2004.19041.572)

Recent messages:
[yas] Prepared just-in-time loading of snippets successfully.
For information about GNU Emacs and the GNU system, type C-h C-a.
Contacting host: www.restek.com:443
error in process filter: mailcap-view-mime: Symbol’s function definition is
void: nil
error in process filter: Symbol’s function definition is void: nil
Making completion list... [2 times]

Configured using:
 'configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY W32NOTIFY ACL GNUTLS LIBXML2
HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS MODULES THREADS JSON PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: ENU
  locale-coding-system: cp1252

Major mode: Fundamental

Minor modes in effect:
  pyvenv-mode: t
  shell-dirtrack-mode: t
  global-linum-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
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nnheader gnus-util rmail rmail-loaddefs rfc2047 rfc2045 ietf-drums
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esh-io esh-arg esh-module esh-groups esh-util elpy-shell elpy-profile
elpy-django s elpy-refactor python tramp-sh tramp tramp-loaddefs
trampver tramp-integration tramp-compat shell pcomplete parse-time
iso8601 time-date format-spec ido grep compile comint ansi-color files-x
etags fileloop generator xref project ring cus-edit cus-start cus-load
wid-edit linum material-theme finder-inf package easymenu browse-url
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syntax facemenu font-core term/tty-colors frame minibuffer cl-generic
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Memory information:
((conses 16 286983 9790)
 (symbols 48 22710 1)
 (strings 32 96281 3012)
 (string-bytes 1 2696643)
 (vectors 16 30701)
 (vector-slots 8 394315 14882)
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 (intervals 56 410 0)
 (buffers 1000 15))