To download a PDF in Meta, click on the paper title. On the right side of the screen, click 
Get PDF, then select Institution, Publisher, or PubMed from the drop-down menu.
The Institution option allows you to access the full text using your institution's permissions. Publisher will take you to the publisher page for full-text options, and PubMed will take you to the paper's page on PubMed and any options they offer.
Please note: To access full-text papers through your institution, you need to link your account to your academic/institutional email [See Link your academic email for access to full-text papers].
For all preprints and open-access papers, when you select 
Get PDF, the PDF will open, or you will be taken to the publisher's page for access.