Why does a red box surround the page when I try and edit my document?

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 functionality you will be able to modify text. 

Click here to learn more about OCR.