Access to "URL extensions" property of page's url from Kentico API

Use the following properties: "DocumentUseCustomExtensions" and "DocumentExtensions"

See examples:

https://docs.kentico.com/api9/content-management/pages#Pages-Pagemanagement

and use:

page.SetValue("DocumentExtensions", ".pdf");
page.SetValue("DocumentUseCustomExtensions", true);

although I'm pretty sure you can set them directly

node.DocumentExtensions = ".pdf";
node.DocumentUseCustomExtensions = true;