Sometimes browsers try to be too smart. In-browser PDF previews might work for some, but other users require the power and functionality of a dedicated PDF application, such as Acrobat. It is possible to disable the built in PDF viewers of browsers, you just have to know how:
In Firfeox, type about:config into the location bar. When prompted, press the "I'll be careful, I promise!" button. Search for pdfjs.disabled and set the value to true by clicking on it.
In Chrome, type chrome://plugins/ into the location bar. Disable the Chrome PDF Viewer and it will become greyed out.