seems pretty easy to me. Just click on the link with the PDF. If it is not readily apparent, click on the "all 2 versions" and then select the one with pdf. In some cases, tho, there will not be a pdf - you can still add the paper to paperpile - it just won't have a pdf. Further, GS does not do an extensive web search for the article... Many times if you google the title, non-scholarly sources will have a pdf. If you have added the paper to PP, once you go to the new website (with the PDF), PP remembers your last addition, and will default to that paper (when you add the PDF).