This ultimately depends on the size of the text. If it is a large piece of text, for instance a street sign, you would have to be far away to achieve better results. If it is a small piece of text, such as a visiting card, the camera and the text you are trying to read could be closer together.
If you are trying to read text on an A4 document, place the document on a table, stand up, bring the phone to your chest height pointing perpendicularly down. This should (more or less) ensure the entire page is in the frame.