When you only pass a string to CreateDocument (even if it is wrapped in Style ), Mathematica creates a new document with a string inside a TextCell . This is why you see the letter string "a\n\tb\n\t\tc" .
On the other hand, your formatted result is Output . Therefore, you need to indicate that a new document should be created using TextCell , which is formatted in the "Output" style.
Below is what you want:
CreateDocument[TextCell[text, "Output"]]

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