The portrait format of most PDFs is one of the reasons I always insist on having a portrait mode monitor. My current setup uses three 4K displays: my ThinkPad

The portrait format of most PDFs is one of the reasons I always insist on having a portrait mode monitor. My current setup uses three 4K displays: my ThinkPad P1's built-in display plus two 24" 4K monitors (currently LG, but I've also used Dell). One of the 24" monitors is above the ThinkPad display, in landscape mode with the left edges aligned. (Actually the monitor's left edge is just a pinch to the left of the ThinkPad display's, so they look aligned when I'm using them.) The other 24" monitor is to the left of these, in portrait mode. The bottom edge is about 3/4" above the desk, allowing room for cables to go underneath it. The portrait mode display is perfect for reading PDF files, and also great for HTML documentation, long text files, etc. Note that 27" monitors would be less ideal for this configuration. The top of the portrait mode 27" display is too high up and would cause neck strain. All three displays are the same distance from my eyes, about 20", and I wear single-vision prescription glasses adjusted for that distance. The ThinkPad display scaling is set to 300%, and the 24" displays at 200%. It is a pretty great setup.