Welcome to the 405th! .pdo is a filetype associated with the software Pepakura, which allows you to take 3D models and split them apart into net-shapes to then use for the pepakura (papercraft) or foam methods. Armorsmith is another program which can read the files, and has similar features, with the added ability to scale and attach them to a digital avatar of yourself.
Pepakura Designer is free to use, except for saving - you'll need a license for that. This means you can import .pdo files and unfold them, but you won't be able to save any progress without paying for it.