Uploading PDF to Amazon S3 and display in-browser

I am uploading PDF's to AmazonS3 manually, using Panic Transmis and via a PHP script/API. For some reason, some display in your browser, and some force download. I have checked permission and can not seem to see any issues, Can anyone help explain how to make PDF's always display in browser ( unless the user specifies otherwise ).

I don't think it is as browser issue.