You can embed links directly in the Description column, as can be seen in the images here. You can have multiple links, a link and a video, multiple links and a video, links and text, etc., as long as you separate them using a space.
If you'd like to format the links as buttons, so they look nice (here, in the ballot on the left, you can see "Click Here to View Poster") means adding a bit of html.
The Description section for you might like this (I put cnn.com where the link to your pdf/website will be- it's just a placeholder):
[Click Here to View Poster|link](www.cnn.com)
You can also change the text in the button to say "View Poster" or just "Poster" or whatever you like.
Here is the general formula:
Please note there is a vertical bar | between "Text" and "link" above. The only fields to change are Button Text and URL.