This is a browser setting and has popped up quite a few times before on the forum.
Basically your client’s browser is configured always to download PDF documents. Getting them to install the latest Adobe Reader should fix that.
https://get.adobe.com/uk/reader/
As for s3-eu-west-1.amazonaws.com
this is Amazon Web Services, our storage provider… nothing to do with hacking
EDIT:
Here are some similar posts:
http://community.quickfile.co.uk/t/pdfs-and-receipt-viewer/6825
http://community.quickfile.co.uk/t/receipts-not-displaying-in-firefox/149