Many lawyers have both a desktop and a laptop computer. Usually both run Windows. If that is you, here is how you can leverage that laptop investment by turning it into a second screen (more or less) for your desktop PC while fully preserving its stand-alone functionality when using it on the road. And this comes free from that surprisingly innovative company - Microsoft (?!).
Download and install Microsoft Garage's Mouse without Borders app first on your desktop, then on your laptop. Then follow the on-screen steps to link the two computers. Now, your desktop keyboard and mouse can also control your laptop. This is essentially a software version of a hardware KVM switch.
OK, this isn't exactly using your laptop as a second screen, but it is close, especially if your files are stored on Microsoft's useful OneDrive Cloud-based storage. That way both your desktop and laptop have access to the same documents. You are also likely to have your key software, such as MS Office and Acrobat, installed on both machines, so you can run the same software if need be.
I use Mouse without Borders to keep my email visible, but not directly in my face, on the smaller laptop screen sitting to the left of my dual 27 inch monitors connected to my desktop PC. Once you move your mouse pointer to the laptop's screen, the software tells your keyboard to control the laptop. When you move the mouse pointer back to one of the desktop's monitors, the software tells the keyboard to resume controlling the desktop PC. Simple, but amazing!
Performance is outstanding. I've noticed no lag in clicking the mouse or typing on either machine. My desktop is hardwired to my router, but my laptop is connected via Wi-Fi, so even Wi-Fi connections seem to work well with Mouse without Borders.
I realize that not everyone uses Windows on both their desktop and portable computers. There are cross-platform choices. This Reddit post discusses other options such as Synergy (Windows, Mac, and Linux) and Share Mouse.