The easiest way to view the downloaded PCBs file is by using the same software they were created with - Eagle. You can download a free version here:

The free version has some limits on the number of board layers and size. However, you should still be able to view any of the downloaded PCB files.

Board and Schematic Files

Eagle splits the PCB design in to two distinctly different files:

.sch - This is the schematic file. It defines the overall electronic circuitry for the device.
.brd - This is the board file. It defines the physical board layout, including its shape, routing electrical traces, pads, silk screen, etc.

These two files are bonded together. Changes in one directly affect the other. For basic viewing, this isn't too important. Just keep in mind that typically both files are open at the same time.

Opening the PCB Files

Here's an example:

Start Eagle and it will bring up the Control Panel.

From the drop down menu, select:

File->Open...->Schematic...

Navigate to where you extracted the files and open the .sch file.

The schematic will open and look something like this.

The board file will also open and look something like this.

Basic Navigation

What you do next depends on what you're after. We'll cover some common use cases next. But if you just want to look around, basic navigation is pretty easy.