I got caught out with this myself a few weeks back and have had a few other people ask me about it since. I am guessing that Google have pushed an update to chrome that has caused this.
Anyway – to turn it OFF goto this site in your chrome browser chrome://plugins/ click the photo below to see what this looks like.
You will see in the above photo there’s a button for Enable under Chrome PDF Viewer, you will most likely find yours will say disable. All you need to do is click disable and it will turn of the Preview feature for PDF files and then download them to your computer instead of showing you the document.