To change default pdf, e-mail and print icon in Joomla article layout, go to the Joomla installation sub directory find & replace these files:

/images/M_images/emailButton.png
/images/M_images/printButton.png
/images/M_images/pdf_button.png

Find icons at http://www.iconarchive.com or at www.iconfinder.net

Check mine icons: