Before, the documents all used to have the same content due to a bug
in the concurrent conversion. I fixed this by creating a function,
'convert_file()', that converts an individual document. I then called
this function concurrently for each file that I want to convert. To
prevent race conditions, where two parallel conversions access the same
file, I ensured that each file gets placed into its own unique directory,
with a randomly-generated name.