从另一个类Swift调用一个类的函数

I have one view controller and it contains two Containers. First Container is embedded with FirstViewController which has 2 buttons and second Container is embedded with PreviewViewController which has webview. I want to load pdf in that Webview. I have taken outlet of webview. on ViewDidLoad Webview is hidden. Now I want that If I clicked on any button that webview sholud be shown and I click Button1 webview should load PDF named 1 and by clicking on Button2 webview should load PDF named 2.I have one view controller and it contains two