When a document is scanned and exported as PDF, most tools create a single image for each page, and no textual information is present. This can also happen when a document is exported to a professional graphics tool such as Adobe Photoshop. Any further edits are then rendered 'on top' of this base image. The Overlaid text signal will display whenever we've identified a base image containing large regions of text, on which further text has been overlaid. 

Clicking on the Overlaid text signal will display each page where overlaid text has been identified, with the corresponding regions highlighted in red. When key details like amounts and dates have been overlaid on a document, it's a clear sign that the document has been tampered with.