Using the below part of code inside your htaccess file, you restrict the browser to download the file instead of displaying it in a seperate tab.

Two versions of the code, one for many filetypes and the second just only for pdf:

Various file types:

<FilesMatch "\.(?i:doc|odf|pdf|rtf|txt)$">
  Header set Content-Disposition attachment
</FilesMatch>

Only pdf:

<FilesMatch "\.(?i:doc|odf|pdf|rtf|txt)$">
  Header set Content-Disposition attachment
</FilesMatch>

taken form the drupal comnunity forum