Sometimes you want to be able to share your project as a PDF or get images to insert them into another document (e.g. a Word document). To do so, click the gear button in the toolbar and you'll get the option to export to a PDF or PNG image.
If you choose PNG, you'll get a zipped folder with each page as a separate PNG image.
Note that if you don't specifically need an image or a PDF, we'd recommend that you simply share the link to your wireframe with your teammates to get the benefit of collaboration, commenting, and preview mode with linking.
Export to JPG isn't directly available, but if you choose the PNG option you can convert your PNG images easily to JPG.