A red box around a document element means it is an image. When a red box surrounds the entire page of a document it usually indicates the document has been scanned. A scanner creates an image representation of the page. Converting this image to a PDF creates a document where each page becomes one image.
Using the OCR (Optical Character Recognition) functionality you will be able to modify text.
Click here to learn more about OCR.
If the OCR module is not available for you, you can purchase it here.