Try to Dowload files getting Error 400

It looks like the different clients are producing different requests. For the requests that are failing with a 400 status code, print out the raw response body. The response body should contain a more useful error message indicating what the issue is.

For the call where you are getting that "%PDF-1.4..." data back, that indicates that the call succeeded, and Dropbox is returning the requested file data. The /2/files/download endpoint returns the exact bytes that are stored on the Dropbox server, which in this case is for a PDF file. From there, you can do whatever you need to with the data, such as passing it to some PDF viewer. I can't offer help with using the PDF data/format itself though as that's outside the scope of Dropbox API support. You may need to check the documentation for whatever PDF viewer you're using, or make sure the file originally uploaded to Dropbox is the version of the PDF you're looking for.