How do I set the default PDF reader? What if I clicked something I am not sure of and the program I chose is not a PDF reader?

For Windows 10, click on the Windows Start Menu on the bottom left corner, then click “Settings”. Next click “Apps” and “Default Apps”. Click “choose  default apps by file type”. This may take a moment to populate. Scroll down to .PDF. Across from .PDF, you will have a list of programs. Choose a PDF reader of your choice.