Removing all stopwords from a list of words
If stop is a list containing \$s\$ stopwords, and words is a list containing \$w\$ words, then the loop in the list comprehension will be \$O(w s)\$, since it basically has to iterate over both lists in a nested loop.

However, if you make the stopwords into a set…

stop = set(stopwords.words('english'))


… then each lookup can be done in \$O(1)\$ time.  You would get \$O(w)\$ running time just by changing the data structure like that.

Another minor issue is that by convention, Document should be lowercase, because it is a variable rather than a class.