Reminder: password protect PDF files.

pdftk to the rescue!
pdftk inputfile.pdf output outputfile.pdf userpw somepassword

If Plasma's network manager doesn't display available networks after suspend/lock screen/etc....

sudo service networking restart
sudo service network-manager restart
Bingo.

Damn you, kscreen

Actually, I like kscreen. The desktop display configuration interface is clicky, draggy and intuitive, and apparently they store settings for specific external monitors by EDID and feed them into xrandr once you connect the monitor.
Enter my Zenbook, various microHDMI to VGA/HDMI dongles, external monitors and projectors. What happens, usually 10 minutes before an important presentation, on a bus without internet and my boss getting antsy, is KDE presenting me with a black screen and the mouse pointer stuck at the top screen edge. Panic mode fix: create a new user from the terminal. In the end I was stuck with 3 different temporary user accounts and no graphical interface unless the µHDMI to VGA dongle was plugged in. Don't judge me, I was overworked.
Still, the troublesome configuration had to be somewhere in my home directory, so I needed to do some more creative renaming, i.e. .kde to .notkde, restarting sddm and praying to the deity of the moment.
Kscreen's config files happened to be in ./local/share/kscreen, I just deleted them all, problem solved.
Got a kernel update in the process and now I need to consolidate user accounts…

Update: As of 09/2020, kscreen still has problems with dongles. (PD USB-C hub with HDMI, XPS13 with BenQ screen) I got sick of constantly nuking the config files and tried apt remove kscreen. Config files still got written. Turns out you have to disable the kscreen service (System Settings->Startup). And now I'd better get used to Xrandr.

Three KDE5 quirks bugging me most right now

I recently upgraded to Kubuntu 15.10 (16.04 by now) and Plasma 5.
Plasma 5 is gorgeous, but still a bit experimental in places. Erm, make that six and counting.

• Baloo desktop search. Once the index is written (takes hours on first run), it's actually usable. However, the config dialog in the system settings is incomplete. You're better off editing ~/.config/baloofilerc, as explained here:
  https://community.kde.org/Baloo/Configuration
  Also, entirely disabling baloo search knocked out Dolphin's nice in-browser file search.
• Dolphin hangs on startup in the home directory when hidden files are set to visible and Kile is running at the same time. (caused by Kile's  ~/.lyxpipe.in and ~/.lyxpipe.out) Apparently fixed upstream: https://bugs.kde.org/show_bug.cgi?id=352828
• The device notifier has lost the automount configuration. There's a hack using the kde4
  versions:
  Edit /usr/share/kde4/services/kded/device_automounter.desktop using the following line (value was 1 before): "X-KDE-Kded-phase=0"
  Use kcmshell4 kcmkded and kcmshell4 device_automounter_kcm to enable and configure automounting. Apparently fixed upstream as well.
  Source: https://forum.kde.org/viewtopic.php?f=289&t=126846
• Dolphin context menu is missing the Ark extraction services. Fixable by sudo ln -s /usr/share/kde4/servicetypes/konqpopupmenuplugin.desktop /usr/share/kservicetypes5/
  Source: https://bugs.launchpad.net/ubuntu/+source/dolphin/+bug/1499530
• Dolphin etc. doesn't remember changes in mime type associations. Apparently it reads from one mimeapps.list file and saves to a different one. The solution is to link both to the same file.
  ln -s ~/.local/share/applications/mimeapps.list ~/.config/mimeapps.list
  Source: https://bbs.archlinux.org/viewtopic.php?id=202636
• Dolphin doesn't display PDF previews, even though they can be enabled in Preferences with kdegraphcis-thumbnailers installed.
  Workaround: sudo ln -s /usr/lib/x86_64-linux-gnu/plugins/* /usr/lib/x86_64-linux-gnu/qt5/plugins/ Sources: https://bugs.kde.org/show_bug.cgi?id=351913 http://osdir.com/ml/ubuntu-bugs/2016-04/msg19852.html