There are also several standalone file openers, graphic tools, and services that open PNG files. A few popular ones include XnView, IrfanView, FastStone Image Viewer, Google Drive, Eye of GNOME, and gThumb.
Considering the number of programs that open PNG files, and that you very likely have at least two installed right now, there's a very real chance that the one that's set to open them by default (i.e. when you double-click or double-tap on one) isn't the one you'd like to use.
The default Windows Photo Viewer program is oftentimes used to open PNG files because it's included as part of a standard Windows installation, but there are manyother ways to view one.
All web browsers (like Chrome, Firefox, Internet Explorer, etc.) will automatically view PNG files that you open from the internet, which means you don't have to download every PNG file you want to look at online. You can also use the web browser to open PNG files from your computer, by using the Ctrl+O keyboard combination to browse for the file.
Probably every single image file converter that you run across will be able to convert a PNG file to another format (like JPG, PDF, ICO, GIF, BMP, TIF, etc.). There are several options online PNG converters like FileZigZag and Zamzar.
Another option for converting a PNG file is to use one of the image viewers I've already mentioned. While they exist mainly as "openers" of various image types, some of them support saving/exporting the open PNG file to a different image format.