Pages.app cannot open or work with just any file you have. Pages opens .pages files and opens via translation .docx, .rtf, .& txt. It will not open any image files such as PDF or jpg or png. Those file types can be inserted via drag-and-drop on to an existing Pages document. As a general rule, Pages will not open any other text files.
You need to use programs designed to open the file type you are wanting to open. You can see what apps you may have on your Mac that can open a file. For instance, I have a .png file & selecting it, right-clicking & choosing Open With… from the contextual menu gives a choice of several apps I have that could possibly open it. By default, the choice is Preview.app. But, as you can see from this screenshot, my Mac lists quite a few apps. The Mac is pretty good at guessing which applications can open a certain file but it's not always right. I don't think that ColorSync Utility, QuickTime Player or Receipts apps can open a .png file.
