The QnA Maker offers up to three (3) managed documents per month for free, if you have more, then there will be a cost, as said over at the Azure free account FAQ page:
https://azure.microsoft.com/en-us/free/free-account-faq/
You'll also find this information over here:
https://azure.microsoft.com/en-us/pricing/details/cognitive-services/qna-maker/
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