Is that Mbps or MB/s? Note that testmy.net provides both numbers.
If it's MB/s, then you should be able to upload an 850 MB file in under five minutes. But if you're only getting 3.5 Mbps, that might be too slow for a file that large. We are currently working on ways to improve the upload experience.
Lastly, can you try to just upload the file to your cloud.acrobat.com account? Does that work?