You wouldn't use a document for this, but instead a SharePoint list. So your columns for your list could have a Subject column, with all of the possible subjects, and a Response column for the canned response you'd send for that subject. Utilizing parallel conditional branches, you would send the Response emails that match the received email's Subject. The Flow would look something like this (just add branches for each response you need):

You could also nest the conditions, it is kind of up to you. I like doing parallel branches at the highest possible level though, to improve the overall performance of my Flows.
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