javascript - what is Filter/FlateDecode and how to deal with it?

There's more context on the background of my project and the code I'm using here. I'm working on this project where we have to store/load pdfs in/from mongo. On the backend, I pipe the output of the download stream directly into the response and on the client side I get a responseText(you can see it in the picture in the linked post) of some weird,unreadable format(Filter/FlateDecode). I need to convert the responseText into an ArrayBuffer so I can feed that into PDF.js to render it in the browser. Can someone please give me some info on type of the string that's in the responseText and how to convert it into an ArrayBuffer?

Thanks

via ribarcheto94