A bit of a late reply but you need to enable 'allowpdfembedding' in [Plugins / Activity modules / File]
/admin/settings.php?section=modsettingresource to allow pdf's to be opened in some other way than downloaded
Totara Learn Open Discussions
How to display PDF files directly in Totara Learn not of downloading them
Re: How to display PDF files directly in Totara Learn not of downloading them