Once you have linked PDF to your Google Drive account, Paperpile will store your paper PDFs in a Paperpile folder called "All Papers" in your Google Drive. The PDFs will be stored in alphabetical folders. See below.
You can add references or PDFs from within Paperpile by clicking the "Add Papers" button in the upper-left hand corner. You can search online, upload PDFs, RIS or Bib files, import your entire library from Zotero or Mendeley, add references manually, or browse other databases.
When you have Paperpile active in your Chrome browser, the "Paperpile" button will appear on Google Scholar, Google and Google Patents beneath each reference. All you need to do is the click the "Paperpile" button to add that item to your library.
Once you've clicked the button, it will change to let you know the paper (and any PDFs) have been added.
Once you set up the Browser Integrations (see image below) in your Paperpile settings, the Paperpile button will appear on many scholarly websites.
In the image above, checking the box next to IEEE Xplore allows your browser to give you the following: