Every email client has its own way of displaying attachments. Based on the file size, the operating system, and the client itself.For instance, web-based Gmail shows the receiver a clear thumbnail of the attachments. Apple Mail yet tends to show the full PDF in the email itself. As most email clients display a thumbnail and its name, be sure to give your PDF attachment a proper name; ‘noname.pdf’ or ‘43j32nfhd.pdf’ is just sloppy.