To view any supported file in Okular, select File → Open..., choose a supported file in the dialog and click Open. Your file should now be displayed in the main window.

If you have already opened files in Okular before, you can quickly access them by selecting them in the File → Open Recent menu.

Okular is the default KDE application for PDF and Postscript files, launched when you click with the left mouse button on such a file type in the filemanager. If you want to open any file whose format is supported by Okular use Open with... → Okular from context menu in the filemanager.

After having a file opened you probably want to read it and therefore navigate through it. Go to the next section to learn more about this.