I think you need to
1) explain what you mean by JSON format and or check the definition.
2) explain what you'll be doing with the JSON data that 'represents' a file
you could, for example, do something like this :-
"MyFileStringArray" : ["File Line 1", "File Line 2"]
but that really depends what you want to DO with the JSON data - you could parse/split each line into some sub-lines/segments etc, but still, that really depends on what you need/want to do with the data
'large text file' ? .. ok - you do realise that the JSON file is going to be larger again, dont you ? - I'd argue if you just want to scan a file line by line, converting to JSON isnt the correct approach - if you're indexing the file, then you may have an array of 'index elements' where each may contain a line number from the file and index/key words
please explain more about your problem (its better if you edit/update your original question)
[edit] btw, you can apply LINQ to text files to scan/split them as well, without the need to convert them to JSON [/edit]