GraphQL cannot return binary data such as PDF or TAR files
(https://stackoverflow.com/questions/43473424/how-to-send-binary-data-back-to-client-using-graphql).
One workaround is to return a URL where one can HTTP GET the binary file.

I think it'd be useful to be able to serve PDFs of some documents (RnRS,
documentation) from the API eventually. There is currently no standard
place where to get Scheme PDFs and the API is as good a place as any IMHO.