When it comes to scanning a document with the Envision App, it totally depends on the size of the text. If it is large, like a road sign, the camera can detect it from far away. However, if it is small, like a business card, the camera needs to be close to the text.
As a rule of thumb, if you are trying to read the text of an A4 document, place the document on a table, stand up and bring the phone to chest height pointing perpendicularly downwards. This should ensure, more or less, that the entire page is in the frame.