My first opinion would be managing a docx in memory should be feasible. On the other hand docx used to launch attacks like this one don't tend to be super big, there isn't any advantage launching attacks using a super big docx I guess (maybe I'm wrong). 

Anyway, I guess you've experienced trying to handle docx's in memory and you're worried about it. If it's the case feel free to share your test cases, please, docx documents causing issues, memory consumption, etc. And I can give a look and ask a more experienced developer to share also opinion :) Thanks!