Document Generation API returning blank document except for hyperlinks

I am attempting to use the document generation API to generate a PDF file from a tagged DOCX file and some JSON data. I using the REST API's to upload the docx asset, then request the merge, polling the returned location, and then downloading the from the given url. The PDF that I download at the end of the process has the correct number of pages and also contains the hyperlinks that the original DOCX file contained. However, apart from that the pages are completely blank - even the hyperlinks do not have any text associated with them. The PDF also contains a number of warnings in the console when viewed. There are two kinds of warning - "getHexString ignoring invalid character", and "invalid stream: FormatError: Bad FCHECK in flate stream". I am unsure what has caused this because all the API calls I made returned success codes and seemed to be okay.