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>