Hi Sciuridae,
this is the normal behavior for this plugin upload_file. The plugin create this link. Once clicked this link starts the controller form SEBLOD and the controller.php starts the download.php, boths scripts you can find in the directory: yoursite/components/com_cck/
In the donload.php you can see at line 13:
'pdf' => 'application/pdf',
This means the browser decides according to its settings.
You can change line 71 from donload.php, but I do not recommend this.
From:
header( "Content-Disposition: attachment; filename=\"$name\"" );
To:
header( "Content-Disposition: inline; filename=\"$name\"" );
in order to force to display the file in the browser.