One way to go about is by using the 'Get OCR Text' action which requires you to provide a UI element. One simple option is to loop over all images, then open them in an image viewer of your choice, and then use the Text -> Scrape Relative to extract your invoice number into a variable.
Desktop Recorder can be used for performing all of the above actions . You will have to adjust the Selectors, as the Recorder usually considers the text the application title as well . You can just open another invoice in your application, then open the Selector and Attach to Live Element to let UiPath work out the Selector for you - which works just fine in most cases.