Make PDF Files Download and not Open in Browser

Most browsers have an inbuilt capability to execute certain types of files. When these types of files encountered in a URL, typically when you click on a PDF Document or Attachment in Salesforce, the browser executes them and displays them in the browser itself, without triggering a "Save As" popup.

For Example, <Instance>/servlet/servlet.FileDownload?file=<DocumentID>

However, in Salesforce, you can choose the default behavior of such kind of files.

From the Setup Menu, Navigate to Security Controls -> File Upload and Download Security.

This page lists the type of files  and their Default Download behavior. There are 3 options available :

a) Download (Recommended): The file is always downloaded.

b) Execute in Browser: The file is displayed and executed automatically when accessed in a browser or through an HTTP request.

c) Hybrid:  The file uses the default browser execution behavior but Salesforce CRM and Chatter files are downloaded.

Official Documentation at https://help.salesforce.com/apex/HTViewHelpDoc?id=admin_files_type_security.htm