You can open up a .pbiviz file using an archive tool such as 7Zip. However, the challenge you will have is that the resulting code is much different to the original source. It has been run through the TypeScript compiler into JavaScript and also minified and assembled via Webpack in pbiviz package to make it as compact as possible for production use, which will also make a number of optimizations and changes. The resulting JS is also encoded so that it can sit inside a JSON property in the .pbiviz.
Even if you can extract and "uglify" it, it is not going to resemble the original source so a lot of guesswork would be required if you were trying to get it back to how the author might have written it.
JSON data from the visual project such as capabilities should be pretty much as entered in the original project though, so the amount of success you're going to have is going to vary based on what elements of the original code you want to look at.
If you can obtain the source code from the author, it will save you from a lot of pain. If you can advise the visual (if a marketplace one) I might be able to assist you with locating the author's repository (providing they have opted to open source their code)
Regards,
Daniel
