Using Plasma 5 with KDE on Linux Mint 18, I have an application that displays a generated PDF in a frame in the app. When I need to move around in that frame, on left click and drag, the window manager grabs the mouse, displays the compass icon and moves the window. I am unable to determine how to disable this, so that I can scroll around inthe document. Setting the left mouse button to do nothing has no effect on this behaviour.

System Settings->Application Style->Widget Style->Configure->(General->)Windows' drag mode.

The default is "Drag windows from all empty areas", and this is what probably causes your problem.
Try to set it to something else.

Completely fixes the behaviour. Appreciated!