Split CSV file into a text file per row, with whitespace normalization
It seems odd, right now, that you throw away the document_id, and that's at least worth an explanatory comment.

The code that transforms each document — the whitespace processing — should be moved into a separate function. This because it is a distinct operation from the CSV reading, and allows it to be separately unit tested.

The code needs to handle the header row from the input.

If by "better" you are asking for faster, well for that you could split up the input file and run multiple copies of your program (and changing how output filenames are generated, since the line numbers wouldn't be the same).