Charlie can't print an attachment, but he can print other things. The attachment is zipped and when he unzips it, it's a PDF file. Leo says that in order to print PDF files, he'll need a PDF reader. Windows didn't come with a PDF reader until Windows 8, so most people wound up getting Adobe Reader. Leo says to open it in Google Drive, and he'll be able to print from there.
There are other PDF readers Charlie could get, such as CutePDF or NitroPDF. But it's critical that any PDF software be kept up to date, because otherwise it could leave his system vulnerable to attack through a malformed PDF. That's the benefit to using Google Drive instead.