No, it is not ambiguous, it is your meaning (b).
If just one person says "yes" you have the answer to this question, and the others might not answer. For (a) all 10 people to have downloaded it, they would all have to say "yes."
If you want to know if (a) everybody downloaded the file, you have to ask a different question. Suppose you ask
Did you all download the file?
then it might be the case that person A already knows that the other 9 have downloaded the file, and answers "yes" for them all. But you don't know that, so you still don't have a clear answer.
So I suggest asking
Which of you has downloaded the file?
And wait for answers from each person, engaging eye contact as you look around them.