A PRO file is a mockup of a website or mobile application created by Adobe Proto, a defunct Adobe Touch app. It includes one or more UI layouts for a webpage or mobile app, each of which may include interactive elements. CSS, WebKit, and jQuery technologies are supported in PRO files.
Brief History of PRO File Format
PRO files were created by Adobe Proto that was replaced by Adobe XD. It was used to create prototyping apps for tablets. Common web design applications like Adobe Dreamweaver used these PRO files for creating web and mobile applications.
PRO File Format - More Information
A PRO file is the first step in the development of a website or mobile application. Developers first create digital mockups that define the looks of the application. These also define the overall application workflow and navigation through application screens. These mockups also include a number of navigation elements such as buttons, comboboxes, scrollbars, input fields and several others.